For simply displaying PDF (and a few other formats), I like SumatraPDF. It is pretty lightweight (compared to the crawling horror that is Adobe Reader), and when you re-open a PDF, opens it at the position you were last time(!!!). The latter feature makes it perfect for viewing reference manuals etc.
EDIT²: IrfanView can display many graphic formats and quite a few that are not strictly speaking graphics. Plus, it has some basic image editing capabilities (resize, rotate, ...).
SumatraPDF is fairly minimalistic, it is a single executable file that runs without installation. Foxit has far more features than SumatraPDF, it is more like the slim sibling of Acrobat (no insult intended!). (SumtraPDF.exe is something like 3.5 MB, Foxit's msi package is something like 70 MB, IIRC)
But Foxit never has given me any reason to complain. I have across a few tricky PDF files that took forever to open in Acrobat, and Foxit (and SumatraPDF) handled those without trouble. Plus, it installs a PDF printer, which is convenient on pre-Windows 10 machines.
EDIT: https://www.sumatrapdfreader.org/free-pdf-reader.html
EDIT²: IrfanView can display many graphic formats and quite a few that are not strictly speaking graphics. Plus, it has some basic image editing capabilities (resize, rotate, ...).