Why can't I reopen my PDF file after editing on my iPad Air?

Almost every time my professor shares my a pdf document for me to edit, and I open it, it can be opened without a problem, but when I try to share it, it doesn't let me. When I close it, it never lets me open it again, or even open it on an external app like Adobe Acrobat reader.

I don't know what happens, could someone help me?

[Re-Titled by Moderator]