Hi there
I’m new with 20.04 since a few days. When I open a PDF with Document Viewer it’s not possible to highlight any text. But you can annonate text.
Ctrl + H switch the cursor to an ‘I’ but I can’t highlight text.
BTW Ctrl + A marks nothing! It seems there is no text?
Sorry for my english 
Are you sure it’s a « real » pdf - ie. text + picture ? Or is it a pdf just made of pictures ?
Try to open this pdf file with LibreOffice Draw, or another pdf reader to find out.
Or in Evince look into the properties of the file, if there’s anything regarding fonts.
If it’s only pictures made, then no text to grab…
You are right! It was a pdf without “text” !!!
Shame on me, but thank for your idea …