Why poor display quality in all PDF readers except those from Adobe?

Only Adobe and Foxit (of those 5) have subpixel rendering. This is seen by zooming way in on the screenshot and seeing color fringing. This is because each pixel on an LCD display is made of 3 subpixels: red, green, and blue. These can provide extra resolution. Foxit maybe just doesn't have as good of a rendering method. Hinting could play a part.

I think am pretty sure now that it's one of the rendering/smoothing options in Preferences->Page Display. Try changing them (there's quite a few) and seeing if it makes a difference.

Specifically:

If you change the "Smooth Text: For Laptop/LCD Screens" to "For Monitor", you'll see that it'll look like other programs' smoothing.

Actually, I think Adobe Reader seems to benefit from the fact that it knows well how to render non clearype fonts (fonts not hinted by utilising Microsoft technology). Most PDF's use professional fonts which are provided by type foundries not using cleartype compatible hinting. Since only Adobe implements the specific rendering for all such professional fonts, Adobe Reader is on top perfectly rendering the smoothing effect.

I finally figured it out and I'm not an IT person (I'm a lawyer and I'm stubborn). I have used PDF Converter Pro 5.11 for several years and I love it. I continue to use it with Windows 10 so don't let anyone tell you it is not compatible. I can solve your problem for this program (PDF Converter Pro) but cannot address the same problems with other pdf programs. You might get some ideas from my solution though. So, I noticed that sometimes, pdf scans barely showed the text in PDF Converter Pro while appearing very nicely in Adobe.

I FINALLY found the solution. When you have your pdf document open in PDF Converter Pro, select the Tools drop down menu, then optimize pdf. There are three tabs, General, Images, and Fonts. You can play with the setting here to change sampling and compression. In the General tab, select "leave compression unchanged" from drop down menu. In the Images tab, select "leave original sampling" from the Sampling drop down and "leave compression unchanged" from the Compression drop down menu. Voila! All the text comes through clearly. I never take time to help others when I find solutions.....very selfish of me. Here, I struggled so long and nobody had the answer. I had to help out. Hope someone finds this after all these years as this is a very old thread!