PDF file with missing text that comes and goes

I'm viewing a PDF and text is missing from it - eg 't' from products  Other people view it on other devices and it is complete.  When I close and re-open some text reappears.  I've uninstalled Adobe reader and reinstalled but problem persists.  Other PDFs are missing graphics too. This is crazy as I use PDFs to proofread creative work and this makes it impossible!  Is this a bug?