Strange issue when trying to open a pdf document

Hi everyone, i wondered if you could help.
I recently purchased an online course, in which the materials were sent to me as pdf manuals. The problem i am having is that when i try to open them, my phone/iPad seems to sort of freeze for a few seconds, and then it seems to read them in a really weird way without appearing to read the actual content of the document.
I have never had this before with pdf files in all the time I’ve had iPhones/iPads, so wondered if anyone else had this same issue and why that was? Whilst they do not always read pdfs perfectly, i have usually been able to open and read them with relative ease. I did wonder if maybe it was because there were images in the documents, and perhaps they’re not formatted to be read on an IDevice? This is particularly frustrating issue for me as I’ve purchased this course and was looking forward to working through it, so any help you are able to give would be much appreciated.