[itsme]

@Bronz: You might consider "managing your files yourself". To do that you have to uncheck "Let MobileSheets Mange My Files" in MS Android, in MS Win10 just specify the folder that you want to use for your song files in "Set The MobileSheets Storage Location".
If you do so, MS uses the original song files from where you import them instead of copying them into its internal data storage.
Just one click on the button "Embed Annotations" is required after annotating to save your annotations in the original file, no PDF export / reimport is required.

Please check, if the MS sync functionality still works correctly when you manage files yourself. 
I have no experience with the sync feature of MS as I also synchronize my files myself: on Win10 my files are in GoogleDrive and synchronized with the GoogleDrive app, on Android I synchronize with FolderSyncPro.
I would be interested to hear if MS sync works in such a scenario or if conflicts between the GoogleDrive sync and MS sync are to be expected.