I was studying with my iPad and when I tried to open a document which I was using 2 minutes before, a message appear: "document empty". I can't use another copy because I had this one underlined. Thank you for your time
Would you please provide more details so that we can diagnose the cause of the problem?
What is the iOS version on your iPad? Go to iOS Settings > General > About > Version.
What is the version of Acrobat Reader on your iPad? Go to iOS Settings > Adobe Acrobat > Version.
Where is your PDF document stored? Is it stored locally on your iPad ("Local") or cloud storage (e.g. Adobe Document Cloud, Google Drive)?
When you saw the "document empty" message, were you trying to open the PDF document from "Recent" in Acrobat?
Would you restart your iPad and try opening the document again? If the PDF document is stored in cloud storage, you can go back to the original source (e.g. "Local", the iOS 11 Files app) instead of "Recent" and open it again.