Compare PDF files

[chnapo]

Hey guys (and girls),
I need advice. I need to compare PDF files, text and graphics. Right now I am using photoshop. Let's say I need to compare 3 PDF files. I open all of them, photoshop rasterizes them, then I hit flatten image and duplicate layer and I copy 2 of them as layers on the third file, this way I get 3 pictures overlapping each other. By simply turning certain layer on and off I can easily spot differences between any of compared files. Also, there is "difference" mode in PS that will black out everything and only show differences. BUT - that is the only thing I use PS for. It is pretty expensive when I use it to ONLY compare PDF files. I am looking for an alternative.
I tried affinity software - problem is that any affinity program opens PDF as editable. I need to open PDF rasterized, non-editable, because it must not change. As soon as I open it in affinity, it changes (I don't have all of the specific fonts that are used in the PDF) text formatting and then it is useless to me. Most of PDF comparison tools compare only texts but I really need to compare them visually.
Any advice for me? Thanks