Read locally saved PDF documents in a third party app (i.e. Adobe Reader)

Hello there! I've created my first Android application, which is actually based from an iOS app I created one year ago. There are over 200 PDF documents. The issue is that when I attempt to open the document with an external PDF Reader like Adobe, I receive the following error message: "The document path is not valid" What I need is probably rather simple for an experienced coder --> I simply need for PDF documents to be readable from third party applications. Even better still, if you could manage to incorporate reading PDFs from within the app, that's great. From what I've understood, this isn't how an Android app handles PDF documents and needs the help of a third party PDF app, so it can't be done. Regardless, for this project, I also require a step-by-step tutorial of how you went about making this happen (including the code you put in). Thank you for your time! -Nicholas S.