What are some alternatives?
When comparing Sumatra PDF and PDF Reader X, you can also consider the following products
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.
Okular - Okular is a universal document viewer based developed by KDE.
Evince - Evince is a document viewer for multiple document formats: PDF, Postscript, djvu, tiff, dvi, XPS...
PDF-XChange Viewer - PDF-XChange Viewer is an alternative viewer for PDF files that enables you to view and annotate PDF...
Adobe Acrobat DC - Make your job easier with Adobe Acrobat DC, the trusted PDF creator. Use Acrobat to convert, edit and sign PDF files at your desk or on the go.
MuPDF - MuPDF is a lightweight PDF and XPS viewer.