Hi Gordon,
The backup file (*.osb) is actually just a zip file and should contain all of your songs as separate files inside it. If you are comfortable with using file browsers, etc. you can copy your backup file, rename the extension to .zip and then open it. Your device should extract the contents into a folder that will let you see what is there. If it is empty, that is obviously the issue! If you aren't confident to check this, email me a copy of your .osb backup file and I can check.
PDF files shouldn't be an issue as long as the Android version is higher than KitKat (4.4). Most Fire tablets run Android 5 or above, so there shouldn't be an issue with that.
On the Google Play Store version, there is an open
beta channel you can sign up to which allows access to v6 of the app, which has many improvements over v5.6 that is available on the Amazon App Store. You can also manually download and install v6 from my website (look on the Downloads page). It may not solve your issues, but it certainly has many improvements on the way files are handled. v6 will hopefully soon replace v5.6 on both the Google and Amazon stores.
Let me know about the backup file check first.
Gareth