There's a scan option in Google Drive that works well
Another vote for Office Lens here, I use it regularly and can't fault it. Get very annoyed with Adobe Acrobat trying to get me to pay (as I think @sam_of_london with a new profile picture mentioned) so try to avoid their other products!
It’s a good app I use it a lot for work.
I've tried this and find Notebloc app much better for scanning and converting into pdf. Free version contains ads but they're not too intrusive so I can live with it ;) https://play.google.com/store/apps/details?id=com.notebloc.app
I use Office Lens and I find it excellent. It has a wider set of output forms so you are not restricted to PDF. You can save to One Note, Word and PowerPoint as well as Gallery and One Drive.