I have now discovered when the problem occurs, and a work around, I hope.
First I downloaded some of the pdfs from my flash drive to my ipad, so I had the files in two places.
Now when I open a pdf from the flash drive and expand it so it fills the screen (bottom rh corner button). It is from here the problem occurs. When looking to contract with the same button to see the menu, it closes the pdf, opens the side menu, and then won’t let you select anything.
If you then go to the downloaded pdf , same thing, all ok until looking to contract screen size again, znd it freezes. The go back to flash drive and it works ok until same operation.
it seems flicking back and forth between the two allows usage, but not convenience.