Some of my PDF documents suddenly display no text - help!

Hi - this is an issue I have only just experienced for the first time, although I now see from searching online that it is quite a common issue.

I tried looking at a PDF document on my Mac yesterday, one which I have read before and not experienced issues with - and whilst the text was displayed on screen initially, as normal, it disappeared after a few seconds. I was then left with a 3 page document in which every page was blank!! Where did the text go?!?

There didn't seem to be any options in any of the menus to change anything to bring the text back. I tried everything I could think of - closing the doc, reopening etc. Nothing worked.

I was supposed to be sending this to someone else via WhatsApp, but instead of risking sending them a blank doc, I just took screenshots with my iPhone during the initial few seconds when the text was displaying and sent them the photos. Not ideal.

Has anyone else had this problem and found a solution? Grateful for your replies.