Premium customer here. The question is not related to the slow loading time, we have already resized all the PDFs to the bare minimum and this part was optimised. Our issue is related to the delay of almost 4 seconds between clicking on a PDF link and then seeing a blank page with absolutely nothing on it until the iFrame shows up.
I can imagine tons of users leaving the page once they see absolutely nothing on it for so long. Is there any way to improve this?