I also have been doing annotations on a computer, but not on the tablets. The problem does not seem to be restricted to just those files. It happens with both a Lenovo Moto Android tablet, and an Amazon Fire tablet.

The backup copy is saved to /data/user/0/com.xodo.pdf.reader/cache/url_cache2/pdffilename

All files are saved on the SD card in the tablets.

Quando abrimos o xodo e buscamos o arquivo no drive, o app cria uma cópia local para a edição ao invés de editar o documento original, conforme esperado. Assim, no momento de salvar, o xodo cria uma cópia de compatibilidade em uma pasta local.
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Para resolver esse problema, acesse o arquivo diretamente do drive e mande "abrir com" xodo. Dessa forma, ele edita o arquivo original diretamente da nuvem e para salvar as alterações basta fechar o arquivo no xodo.

I am having the same issue.

I annotate the same document alternatingly on a tablet and a computer, often with large pdf-files (10MB+). I have found Xodo to be the best at integrating with Google Drive in this way. However, I am often met with "permission errors". Only after I am done annotating am I met with the following message:

"Changes to [FILENAME].pdf could not be saved to the original location due to a permission error. Xodo has saved a backup copy of [FILENAME].pdf with your changes here:
/storage/emulated/0/Android/data/com.xodo.pdf.reader/files/Download/[FILENAME].pdf"

This occurs when I access a file on Google Drive from the Xodo app on Android. My hunch is that it occurs when internet access on the tablet is temporarily interrupted or the tablet changes networks. For the last document I annotated, this occurred twelve times. Each time, I have to replace the file on Google Drive with this local backup in order to continue annotating on Google Drive. I would much prefer it if the app had a manual "save" button, kept trying to save to the original location until it worked (at least a few times), or at least asked me if it should try again when saving the file to its original location didn't work the first time, instead of just making a backup copy immediately without asking.

Xodo Version 7.0.13 Build 9.0130566
Samsung Android Tablet
Windows 10

I am having the same issue

Same here for Google Drive on v.7.0.7 and Samsung Galaxy Tab S6.