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App doesn't show in Recents Screen when launched via an intent-filter

I am trying to launch my App (parsing a pdf file and storing the aquired data to a database) via an intent-filter linked to pdf-files. Whenever I start the App normally, it will show up in the Recents Screen.
However, if I use the View Intent/Action from another app, it will still behave as expected but it is not shown as an own page in the Recents Screen. Instead the app from where the pdf file is selected will be shown in the header and the app icon, showing the screenshot from my own app in the Recents Screen page anyway.
I already tried removing the android:label in the manifest file and I don't have the android:excludeFromRecents attribute set.
What other circumstances might lead to the described behaviour?

What other circumstances might lead to the described behaviour?
It is completely normal behavior. By default, started activities go in the same task ("page in the Recents Screen") as the activity that started it. This is similar to how tabbed Web browsers work: links normally open up a page in the same tab as the page that contained the link.
Where you see different behavior is because somebody took steps to change the behavior:
The activity that started yours could add flags to control task behavior, such as FLAG_ACTIVITY_NEW_TASK
Your activity could have manifest attributes to control task behavior, such as taskAffinity, allowTaskReparenting, launchMode, etc.
See the documentation for more.

Lag Android app after some time

Well the question is that I have an app with multiple activities. The main activity runs several animations and the problem is that if I run the app and stay in the main activity everything is all right but if I go to another activity (the setting activity, for example) and come back to the main one, after some time the hole app begins to lag.
Any clue about why is this happenning? May the other activities keep running in background or something like that?
Thank you all.

As far as your scenario is concerned, try following
Set up proper navigation within your app.
If you keep closing and opening your activities (when your activity is still holding on resources), your memory usage keeps piling up. So make sure you release all resources like camera, or finish an animation before calling finish();
Try different android:launchMode options for your Main Activity /Home screen activity. (this depends on your app design)
In order to find who is calling your app to use more memory, try https://developer.android.com/studio/profile/investigate-ram.html
PS. You could've given more details like #Michiyo mentioned.

how to prevent growing activity stack in android

I read somewhere that each time you call startActivity() to transition to a new screen, you are creating a new instance of that activity. This immediately raised a red flag to me.
In search of a way to prevent this issue, I read about using FLAG_ACTIVITY_REORDER_TO_FRONT. However I'm a little alarmed that this method is not used in any of the tutorials I've seen for opening a new screen in your app. So I think I might be missing something. I mean wouldn't you always want to make sure you're not creating a duplicate Activity? isn't this kind of a big deal and shouldn't tutorials address it?
I just want to make sure that I am understanding, and dealing with, this issue correctly, and using the commonly practiced way to transition between screens.
I understand in some cases you open a screen, do something, then close it using finish() and go back. But if an app has a complex 4 level hierarchy , and the user needs to be able to jump around between the screens?

I mean wouldn't you always want to make sure you're not creating a duplicate Activity
Most of the time you probably won't want to have two instances of the same Activity but I suppose there are situations where someone might
isn't this kind of a big deal and shouldn't tutorials address it
It is definitely something that Android developers need to know about and understand how to use. I would imagine that you may not see a lot of it in tutorials because most of them teach you the basics to get you started. When a developer is new to Android, they typically have enough to learn about the framework. And they normally show you how to do a few screens (which is often all someone may need). They expect you to learn more of the details by reading through the docs and using websites such as SO
I understand in some cases you open a screen, do something, then close it using finish() and go back. But if an app has a complex 4 level hierarchy , and the user needs to be able to jump around between the screens?
Yes, a lot of times you won't be getting too deep because the deeper the easier it is to get a "lost" feeling. This is why I like using Activities with a Dialog Theme when possible. It gives the feeling that you aren't actually leaving the screen you are on. However, that doesn't really answer your question. There are many flags that can be used with Intents to keep the stack from growing. The one you mentioned works and I also use Intent.FLAG_ACTIVITY_CLEAR_Top quite often if I need to clear all Activities from the stack and get back to one single Activity. There can be so many different situations between apps, users, and developers that how you transition and work the flow or navigation just depends on what you (really, the user) needs.
It seems like you are on the right track by asking these questions. Think about what will give the user the best and most natural experience and find the right flags in the Intent Docs for your situation
I hope this cleared things up for you a little. If you don't understand something, feel free to ask

You usually want to avoid creating a duplicate activity, and IMHO, it's a bug in the Android system that the default behavior is to allow it.
(Sometimes you do want to allow it, e.g. you've written a "get filename" activity, and more than one application is likely to call it.)
You control activity creation in two places: In the manifest, and in the flags of the intent that launches it.
In the manifest, the <activity> tag has the attribute android:launchMode, which can be one of:
"standard"
Default. Can be instantiated multiple times, can belong to any task, and can appear anywhere on the stack. Normally part of the task that called startActivity() unless the FLAG_ACTIVITY_NEW_TASK was used. A new instance of the class is created to respond to each new intent.
"singleTop" Identical to standard, except that if the target task already has an instance of this activity at the top of the stack, a new activity will not be created. Instead, the existing activity will receive a call to onNewIntent().
"singleTask" There can be only one. It's the first activity of a new stack, and is thus the root of an activity stack. If there are more intents, they'll be sent to onNewIntent().
"singleInstance" Identical to singleTask, except that it's the only activity in its stack. If it tries to launch a new activity, the new activity will start a new task. Same as if FLAG_ACTIVITY_NEW_TASK was in the intent.

I personally find the history back stack in android more confusing than helpful to end users. The use of back buttons can often not do what the user expects.
There are a few options available to you if you want to use activities as the main construct.
For workflow type activities (e.g. capture forms) start the first activity with Intent.FLAG_ACTIVITY_NEW_TASK and at the end use FLAG_ACTIVITY_CLEAR_TASK
For top level activities that are often a returning point start the activity with Intent.FLAG_ACTIVITY_CLEAR_TOP. This basically checks if the activity is already somewhere in the stack and if it is pops off every activity above it in the stack and resumes the original activity.
Another possible method for the top level activities is to use one top level activity and use fragments for moving around, e.g. tab browser etc. Then just use activities for actual separate tasks.

How is back button on navigation bar implemented in AOSP code?

I am trying to understand what happens when you press the back button. how does the system manipulate the back stack? From where does it get the window handle to the top window?
I have looked into the PhoneWindowManager.java, but couldn't find what I am looking for. Has anybody seen this code earlier?

What you are refering to as the "back button in the action bar" is called the "up button". The difference between the two is well explained in that article from the documentation: Navigation with Back and Up.
In short:
The Up button is used to navigate within an app based on the hierarchical relationships between screens.
The system Back button is used to navigate, in reverse chronological order, through the history of screens the user has recently worked with. It is generally based on the temporal relationships between screens, rather than the app's hierarchy.
Difference is well market for instance when you navigate to an app from another app. E.g.: you are accessing an application's Google Play screen from a link in your mailbox. Once in Google Play:
Up will take you to Google Play's home for applications
Back will take you back to the GMail application

Well, this will take you back, as like the back button would:
super.onBackPressed();

The documentation for onBackPressed() says that the Back button simply ends the current activity. No back stack manipulation is needed.
At a deeper level, Android does maintain a stack of activities inside of a Task, and it can determine which activity to show next when the top activity finishes. But those mechanisms live in native code and are probably beyond the scope of what you're trying to do. As far as I know, the best you can do from Java is to use RunningTaskInfo (source code here), which will expose the "base" and "top" activities in the current Task.
If you're trying to implement complex app navigation, there are a number of flags that you can use to control the way in which activities are placed on the stack.

Android, managing views, task and activities

I need general advice.
I'm building an app that can be started via the icon (normal way) but also with an intent triggered by a click in the notification area (the app places an icon in the notification area). The is awaken when a time event / alarm occurs.
The app has 5 to 10 views. Is it better to open a new activity for each view? How can I be sure not to have mutliple tasks open? Say if my app is Activity-A, Activity-B, Activity-C and can be started from Activity-A and Activity-B (depending on if it's opened via the icon or the notification area) ... how do I clean everything upon app close?
Or is it better to have one activity and just switch views (xml layouts)?

I'll start of with what you should watch and read. It should be clear that navigation in Android is very easy to do wrong. Google has a history of doing differently from app to app, but they are getting better. This said, if you are to do an app you should know how it is intended to work, and the best way to do that is to read the docs. As mentioned, there was an excellent talk at IO'12. There is also a very good section on the design site, and finally there is a good section in the API Guides.
To summarize: a full screen is an activity (which in turn can be constructed of fragments). The activities should in virtually all cases be structured as a tree with the root being the activity that are launched from home. For every activity you should have an "up" activity that takes you up in the hierarchy. Note that this is different from back which should take you to the last full screen state you were in. Also note that full screen means that for example tabs should not be recorded in the "back history", since they provide navigation within a screen.
When it comes to tapping a notification it should be equivalent to: pressing home, remove the task from the recents view (clearing the task), opening the task, and finally take the shortest path to get to the activity presenting the info that the notification told you about. Complicated, yes indeed... But at least there is a helper class in JB and in the support library called TaskStackBuilder.
The key to all this is a UI design that follow the Android design guidelines. Take your time to make the design for your app, and make sure to separate up (static) and back (temporal).

It might be as simple as using a different launchmode, which you can define in the manifest or I think in the inent you are using. It takes some experimenting but SingleTask or SingleInstance may be the right choices for you,
Your activity should support onNewIntent in this case, to reuse existing Activities if that is the desired effect.

In this case I better use one activity or use fragments.