A lot of applications claim they can display PDF files, but not all of them fully support the format. They won’t necessarily display all valid files correctly. The PDF Association has an article discussing this problem, with the main focus on the Microsoft Edge browser.
Edge offers only partial support for the JBIG2Decode and JPXDecode filters, which means some objects might not display. It doesn’t support certain types of shadings, so other objects could render incorrectly.
The strength of PDF is supposed to be that it will render the same way everywhere. You can blame Microsoft for not putting enough work into it, or Adobe for making the format too complex. I have enough experience with it to know it’s a seriously difficult format just to analyze, to say nothing of rendering. Is a format which presents such difficulties really the ideal for a universal document rendering format that people will count on far into the future?
Update: It gets worse. Take a look at this discussion of what’s in PDF.