Plainapp as default pdf viewer

[KyrillGobber]

Is your feature request related to a problem? Please describe.
You already got an amazing pdf viewer, please let us pick it when opening pdf files

Describe the solution you'd like
When opening a pdf file from any app, android shows a 'open with' dialog to pick the pdf viewer app. plainapp pdf viewer should show up there.

Describe alternatives you've considered
Currently I'm downloading my pdfs to a folder, then open plainapp and navigate to the document to open it with it