When a file is saved, changes dissapear from the original file

Hi,

One of our colleagues at the office has the following problem when changing textboxes on a pdf file and saving the file.

1) A person makes a change to a pdf like splitting a text box into multiple text boxes.

2) The person then saves the file using ctrl+s or the Save button.

3) When the person opens the file again the changes are gone OR the changes are still there but...

4) the changes are reverted back after a while.

We're unsure what causes the problem, the file was in a shared sharepoint folder, but the problem still persists after moving the file out of sharepoint and into an unshared local folder on the pc.

The person is running Windows 10 pro.

Thanks.