Images in PDF appear blurry on select PCs

Also, the affect of the Page Display Rendering prefs would be more or less obvious with different types of images and the zoom magnification:

Acrobat Reader and AcrobatPro have Page Display Rendering Preferences that affect image and line anti-aliasing. Those settings are controlled by the client:

Oh and one other thing: people often overlook the simple question: do ALL PDFs look bad on your computer. "Oh, yes, all of them, sure... "  

One technical thought: this may be high resolution monitors, on which many apps will display everything at low resolution. That would include older versions of Adobe software, so always check versions too, and ask for monitor spec when you're doing tech support.

There is no difference in the way Acrobat Reader and Acrobat Pro render PDFs; nothing is "downgraded" in the free Reader software, and image objects on a page only have one stored set of pixel data.

In Acrobat/Reader's preferences there is a "show large images" option but all that does is turn high resolution bitmaps into gray boxes, it's an ancient feature designed to make scrolling faster on low-spec computers. You might ask them to try toggling "2D graphics acceleration" but I can't see why that would blur anything.

It's possible (likely) the customers are using completely different software. Most just open PDF files in a Web browser, and let the browser's internal JavaScript-based engine handle it. They are, to say the least, limited in their abilities. However that would not be an Adobe or PDF problem. The whole point behind PDF is that a "compliant" rendering application should always display the page exactly the same (assuming the hardware is suitable).