Can you please share some Sample PDFs with me with which you are facing the issue. ?
Also, It would be great if you could share a Screen recorded video of the whole thing so that we can be sure of the steps that you are doing.
I assume the Documents that user is trying to access would be having some Initial View Zoom Set.
Open any such file with which you are facing the issue, Do Ctrl+D(File→Properties) , Go to Initial View Tab and see the Magnification value.
Check this setting and set it to “Default” and see if that works then.
Choose Edit > Preferences.
Under Categories, select Page Display.
Open the Zoom pop-up menu and choose a default magnification level.
You may also refer to the following link for more information: Adjusting PDF views