You have made some progress; don't be too quick to make conclusions, though. That PDF files render correctly in safe mode suggests that the problem is not macOS 10.13 per se, but perhaps with an incompatible login, user-installed font or other feature that is excluded by safe mode. Check these two Apple Support articles:
https://support.apple.com/en-us/HT201262
https://support.apple.com/kb/PH25614?locale=en_US
The second article explains how to remove all of your login items. After you do that, re-boot and re-check your PDF files in the Preview App. If the blurry file problem is resolved, then re-install your login items one by one (re-booting each time) to find which one caused the problem.
To be even more thorough with isolating the problem, make sure you check with different types of PDF files (i.e., from different sources). Use some PDF files from your own scanner, some created in your Mac by using the "Export as PDF" function or the "Print" then "Save as PDF" function in Safari, some downloaded from other sources, etc.
Another clue to solve your problem is seen in this forum thread. Several users state that they have no problem with PDF files under High Sierra (macOS 10.13). I am in that category as well - my PDF files now render perfectly in Preview in my two Macs - a late 2014 iMac Retina 5K and a late 2010 MacBook Air, both running macOS 10.13.2. It appears that the problem you are having must be due to some specific issue with your setup. Maybe an incompatible login or installed font, as discussed above, or a corrupted macOS installation or other incompatible or corrupted software installation.
BTW, I share your dislike of Acrobat Reader. When I last used it several years ago, it was slow and a pain to keep updated. Preview has become much better in recent years with its added functionality. I hope that Apple will continue to improve and add features to Preview, but I'm not holding my breath.
Good luck ...