Sumatra PDF is an open-source PDF, eBook (ePub, Mobi), XPS, DjVu, CHM, Comic Book (CBZ and CBR) reader for Microsoft Windows OS (runs on XP, Vista, 7 and 8).
It is a small application focused on a clean user interface, simplistic design and with an impressive, fast start-up, which also recommends it for use on less-powerful (older) devices. This PDF viewer is available in multiple languages and its currently offered as an installer (clean there's no bundles, installers or any other form of adware) and portable version (you can run it from your USB drive and it won't write to registry).
Note: Although, the first versions of this program were primarily designed for Windows XP and theoretically could run on some older Windows versions, there is no support or any other guarantees that you will be able to run Sumatra on older Windows such as Windows 95, 98 and Me.
Sumatra PDF Review
Hint: You can read here a longer review of Sumatra PDF (including images).
A little history: Probably, the most popular PDF reader (and the oldest) is Adobe Acrobat Reader considering that Portable Document Format (PDF) file format was developed by Adobe Systems in 1993. Anyway, I installed my first version of Acrobat Reader in 2001 and since then, I've noticed two things: (1) each new released version was bigger than the previous one and (2) I could not afford to change my PC each year to keep up with the new hardware requirements of such programs. Certain software titles, were coming up with new features that demanded more CPU, RAM and more space and as I said, I couldn't keep up with these requirements. To put an end to this idea: 99% of the time, I used Adobe Acrobat Reader to read PDF files and that's it so those new features seemed to slow down my PC were not for me.
Who should use this: I am not a fan of the so-called "software suites" that does 100 jobs and only a few of them works right. This PDF viewer does what his name suggests: it will open fast PDF files (and also ePub, Mobi, XPS, DjVu, CHM, CBZ and CBR) so that you can read them on your PC/laptop/tablet or any other device capable to run Windows. Please note that it doesn't offer advanced features that can be found at other (paid) alternatives such as, PDF editor, OCR capabilities, social media features, advanced export functions for other formats etc.
Basic features: You don't have advanced features that can be found at other similar software but it does offer: PDF browser plugin (for Chrome, Firefox or Opera), print PDF files, show previews of PDF documents, adjust the default layout or zoom, remember opened files and even settings for each document, customize options (for advanced users using the SumatraPDF-settings.txt file), keyboard shortcuts etc.
Conclusion: If you just want to read PDF files, use common basic features, appreciate a minimalistic design and low resources usage, I recommend and rate it as being the best ultra-lightweight PDF viewer.