With respects to PDF I use: SumatraPDF (leightweight display), Foxit Reader, Diff PDF (Comparison), PDF X-Change Viewer (edit, esp. connotate and comment files), PDF TK Builder (merging, watermarking etc.). I also use FreePDF for print-to-PDF purposes, note that this needs a local install as it creates a virtual printer device. Most of these since WINXP, currently all on Win7 Prof. 64-bit and Win 8.1 Prof. 64 bit respectively.