Need sample site so others can replicate the issue or find out what might be the cause of the unreadable site.

That is probably a problem with a font that is embedded in the PDF file or in case of a local font file, no permission to read this file. Firefox shows a purple shield instead of a gray shield at the left end of the location/address bar in case Enhanced Tracking Protection is blocking content. *click the shield icon for more detail and possibly disable the protection You can check the Web Console for relevant-looking messages about blocked content. *https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop *https://support.mozilla.org/en-US/kb/smartblock-enhanced-tracking-protection *https://support.mozilla.org/en-US/kb/trackers-and-scripts-firefox-blocks-enhanced-track

Here is a sample of my problem. If I download this statement as a PDF and then open it as a PDF, all appears as it should.