Foxit Reader is my PDF reader of choice.

Any modern browser will open *.pdf files.

I've yet to find anything that can compare to lightness or simplicity of mupdf.

I'd really love to say yes, but the amount of issues that crop up featuring it clearly shows otherwise. Personally, I stopped years before I switched to Linux, this was back when Foxit was new and cool and not bloated / malware (2007).

Sumatra or bust

I mostly use Evince or some of its forks.

Almost dumped Foxit a while back since it didn't work with PDF forms, but data entry now seems to work after some update or another. Might give Sumatra a try if Foxit causes me any problems.

^^^

This would also be my suggestion. No need for external programs and if the user don't require fancy features any browser will do.

Sumatra or Foxit Reader here.
They both work hand in hand. If you don't like the look of the way Sumatra displays, Foxit probably does it better and viceversa.

For compatibility reason I still have Adobe Acrobat Reader installed, although I mainly use Foxit.

Some of our customers use weird PDF functions that simply don't enable us to print properly from other readers than Acrobat. Perplexing.

this is the same thing Microsoft does by changing XML in there base document handling... thankfully with the move to cloud a good portion of people are steeping away from Office365 and moving to Google or other open format.

Nah, I still use it, for now at least.

I worked somewhere where they used Foxit Reader as a default and I hated it, and since Acrobat is free I've always used it.

I use the portable version of PDF-XChange Editor

3.1.1 (2015-11-02)

Changes in this release:

(re)add support for old processors that don’t have SSE2

Interesting, that it supports CPUs that are about 16 years old or older. So, if you still have an Intel Pentium or AMD K6 or Cyrix PR 166 CPU you can use that viewer. :-D