What are some alternatives?
When comparing Okular and PDF Reader X, 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.
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.
Evince - Evince is a document viewer for multiple document formats: PDF, Postscript, djvu, tiff, dvi, XPS...
MuPDF - MuPDF is a lightweight PDF and XPS viewer.
PDF-XChange Viewer - PDF-XChange Viewer is an alternative viewer for PDF files that enables you to view and annotate PDF...
calibre - Ebook manager, viewer & converter