How can I get the apk file of an app? A while ago, I asked a question which has been deleted by Community


  Some Android apps are in fact ebooks. For example, Learn Lisp
  
  Is it possible to convert android app ebook to pdf or some other document file types? Are there apps to
  do that?


Andrew T. commented that:


  For this particular app: get the APK, open it with zip viewer, and
  browse the /assets folder; they're in HTML files. I can't answer if
  there's an automated way to convert it to PDF because the content
  structure will not be the same for other apps.


Firelord commented that


  @AndrewT. good approach. You can add certain tweaks and I think it
  would make a good answer, well at least for some apps, if not for all.
  In Android, download busybox, get the APK, and then do unzip -l APK_PATH | grep -E "FILETYPE$". If there exists a file with your
  extension, its path in APK would be listed. If the developer went to
  lengths to change the extension and hide the file, then this would
  fail.


My questions are:


How can I get the APK file of an app after installing it from Google Play? 
Once I find the APK file of an app, can I copy it out of my android phone to my Ubuntu laptop and then extract the hidden document file in the apk file? Or do I have to extract on my android phone?


Thanks.
How can I get the APK file of an app after installing it from Google Play? 


Multiple ways, but the easiest one, I suppose, is to install a backup type app such as App Backup Restore. Take the backup of the the target app and its APK would be saved inside a directory under the default storage (/sdcard).


  Once I find the APK file of an app, can I copy it out of my android phone to my Ubuntu laptop and then extract the hidden document file in the apk file? Or do I have to extract on my android phone?


An APK is an archive (MIME type: application/zip). You can copy it to your laptop alike any multimedia file. 

As for the extraction of the contents, there are multiple apps on Play Store which supports extracting a ZIP file, so it is up to you to extract the app on Android or in your PC to explore its contents.