[android tablet] Bizarre pdf problem

[unixman_again]

Our students have Hisense Sero Pro 8 which are generic Android tablets. When one student goes onto Moodle and downloads specific pdfs, she gets an error message saying the pdf can't be downloaded. Other pdfs on the same page download and open fine. I had her try an identical tablet and the first load of pdfs were also fine. I suggested she try a factory reset and see if that cures the problem. I'm completely baffled.