Just saying and I have no idea if this is possible: can't a copy of the original file be created before or when it is modified to protect the original file? With the file name xyz_modified.pdf or something similar. That way the original file would be preserved and modified files would be easy to identify.
Samsung Galaxy Tab S7 FE Android 12
Samsung Note Pro 12.2 LineageOS 14.1
Huawei Media Pad M3 lite Android 7
Samsung Note Pro 12.2 LineageOS 14.1
Huawei Media Pad M3 lite Android 7