Recs.
Updated
Sumatra PDF is a free PDF, eBook (ePub, Mobi), XPS, DjVu, CHM, Comic Book (CBZ and CBR) reader for Windows.
SpecsUpdate
Pros
Pro Portable
The application consists of a single file that can be run without installation. You can put it on an external device, such as an USB thumb stick and run it on any Windows machine from the thumb stick.
If you chose the installer version you can chose options to include previewer with the basic ability to scroll page thumbnails in explorer.
Pro Handles many file types
Including - AI, EPS, PDF, CHM, DjVu, EPUB, PJL, PS, FictionBook2, Mobi, PRC, OpenXML Paper Spec, XPS, XOD and Comic Book Archive Files such as CBR, CBZ, CBT, CB7 and can convert those and other images to PDF.
Image files such as TGA TIFF PNG JPG GIF WEBP BMP (and save them as PDF)
Plain Text files can contain hyperlinks or mailto:addresses
Cons
Con No annotation or editing facilities in the standard version
Requires the Pre-Release for editing annotations such as Highlights or Comments.
Con Fonts are not smooth enough
Not as smooth as e.g. Adobe Reader. MuPDF anti-aliasing is not configurable.
Con Lacking exploitable features
Sumatra aims for simplicity at the expense of functionality.
For example, there's no way to fill in XFA form PDFs with user tracking entries nor edit them.
Simple files that track a user IP whilst reading a page do not work.
Con Awful printing experience
Sumatra sends bitmap files to your printer, I've witnessed an increase in literally (not figuratively) 100x the original file size! If printing multiple PDF files at once is part of your routine, SumatraPDF is not for you, since can easily fill up your RAM. Even if it doesn't, it still is much slower than any other PDF viewer when it comes to printing.
Recommendations
Comments
Flagged Pros + Cons
Con Lacking in features
Sumatra aims for simplicity at the expense of functionality. For example, there's no way to annotate PDFs or edit them.