I just ran into this issue. I sometimes receive a series of jpg images of various documents e.g page 1.jpg, page 2.jpg etc. In the past, I'd just put them together in Preview and save as PDF. All fine.
Today I ran through this again, and WTF - the preview is all blurry! I've tried various Export as .. and Print / Print to PDF options, but to no avail.
Very frustrating. Running OSX 10.13.5.
Are you using an external monitor? on the MacBook Pro it renders very clear, but on the LG monitor is all blurry. With Sierra I don't have this problem, only with High Sierra. I tried Mojave a week ago and pdfs are blurry.