And how are the customers accessing the PDFs?
What devices are they using?
What program or apps with versions are they using?
I this age of other then PDF eBooks formats and reader program and apss for computers and mobile devices for these formats I would look at creating the books in one or more of the eBook formats.
Google Chrome and FireFox both have their own version of a PDF reader plug-in and they are well known to be less capable than Adobe Reader. It is possible to configure these products to use Adobe Reader.