What are some alternatives?
When comparing Okular and PDF-XChange Viewer, you can also consider the following products
Sumatra PDF - Sumatra PDF is a slim PDF/DjVu/EPUB/XPS/CHM/CBR/CBZ/MOBI viewer for Windows.
Evince - Evince is a document viewer for multiple document formats: PDF, Postscript, djvu, tiff, dvi, XPS...
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.
PDF Reader X - PDF Reader X is a nest in the class PDF reader, PDF annotator and PDF editor for the Mac OS X operating systems.
MuPDF - MuPDF is a lightweight PDF and XPS viewer.
Xodo - Xodo is the ultimate cross-platform PDF viewer and annotator. And it’s 100% free.