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.