Preview displaying very low resolution

Suddenly Preview is displaying PDF files in low resolution, sometimes unreadable. The files are ok, press spacebar in the Finder and the file displays very sharp. I copied the files to another Mac with older Preview and they display fine, and also to a PC and they display fine. I deleted the .plist and rebooted. Didn't help. Tried setting the "open in low resolution box" for Preview. Didn't help. This just started in the last couple days. I'm not aware of any changes or upgrades. MacOS Sierra, 10.12.6. Preview 9.0, MacBook Pro.