when I open a pdf file, after some time later, it goes to the first page. It looks it restarts the app.

There's no problem with my Galaxy Fold 6 or S series. It only happen on my Galaxy Tab S8 ultra. As i remember, it also happened on Tab S7 FE. Maybe it happens only in Galaxy tablets.

I reported the issue to Samsung, however their answer was "this is caused by Adobe Acrobat app"

Please check the issue. it bothers me a lot when I read pdf file.