convert PDF to grayscale

gs -sOutputFile=output.pdf \
 -sDEVICE=pdfwrite \
 -sColorConversionStrategy=Gray \
 -dProcessColorModel=/DeviceGray \
 -dCompatibilityLevel=1.4 \
 -dNOPAUSE \
 -dBATCH input.pdf

Command line tool to join multiple pdf files into one on Mac OS X

brew install poppler
pdfunite input-1.pdf input-2.pdf input-3.pdf output.pdf

PDF viewers on Linux

1. evince
2. acroread
3. xpdf
4. kpdf
5. gv
6. okular
7. mupdf

   sudo apt-get install mupdf

8. zathura

   sudo apt-get install zathura

9. qpdfview

   sudo apt-add-repository ppa:b-eltzner/qpdfview
   sudo apt-get update
   sudo apt-get install qpdfview

10. foxit