PDFs Greyed Out When Shared During a Meeting

I have PDF files that I have stored on ICloud that I share during meetings.  Quite often, when I try to share one of these files, the screen is greyed out on top, and miniature copies of the pages across the bottom the screen.  The only work-around I have found so far is to rename the file, either before I start (if I have time to check it on a Test Zoom session) or during the session.  When renamed it shows up perfectly.  Does anyone know why this happens and how to prevent it?