Font Smoothing PDF text in High Sierra
There seem to be different issues at work here (judging by the comments.)


Preview is getting buggier. Using an alternative way to display PDFs might be the only option to get a proper display of text, at least for now. Sometimes just using QuickLook will get you better results than Preview.app. To get around this "fubar'ed bugginess", some even go to lengths: "opening the same PDF in the Snow Leopard VM (running on Sierra) is actually the fastest of the three! What did Apple do to the Preview app in Sierra?"
When scrolling macOS now uses an intermediate rendering to conserve CPU cycles that gets updated to its final representation only when no more scrolling action occurs. That behaviour started at least in Sierra and actually that got a bit better (quicker, less noticeable) in High Sierra. 
Also already in Sierra the default FontSmoothing settings were altered to a different default. That should not have changed in High Sierra (new OS bugs excluded). If the font display is not to your liking then there are the two checkbox settings in System Preferences to play with. 


Solutions


Issue No.1 will only be fixed by faith and prayer and hope in a better future.
Issue No.2 usually should 'fix' itself, once scrolling stops (you may to to release the touch from any touchpad you might use). In all those numerous instances that it doesn't: file more bug-reports (= not with the Feedback.app) (The example in the comments trigger a bug retained from Sierra: slightly substandard PDFs that were assembled including graphics or PDF-files with suboptimal page attributes; might be fixed by reflowing the PDF; has to be done for every PDF triggering that bug, upgrading from Siera-Preview to Adobe Reader is one way to spare the hassle, downgrading to Yosemite the other.) Once the blurriness-stays-after-scrolling-bug is triggered your only workaround option is to quit Preview (really quit: check in Activity Monitor that the process is actually gone) and work with another PDF or use a different Reader.
Issue No. 3 lists the options you have to customise the settings to your liking. If both options from System Preferences are unsatisfactory you might get a more fine grained control with entering the following in Terminal.app:
defaults -currentHost write -globalDomain AppleFontSmoothing -int 2
Where y might experiment with the values for int between 0 and 3. The higher the number the more smoothing (some call it blurring) should be applied.