Xpdf is an open source PDF viewer.
XpdfReader is a free (but not open source) PDF viewer, which includes a few extra features
not found in the open source xpdf viewer.
Command line tools
The open source Xpdf toolkit also includes several command line tools which perform
various functions on PDF files:
* pdftotext: converts PDF to text
* pdftops: converts PDF to PostScript
* pdftoppm: converts PDF pages to netpbm (PPM/PGM/PBM) image files
* pdftopng: converts PDF pages to PNG image files
* pdftohtml: converts PDF to HTML
* pdfinfo: extracts PDF metadata
* pdfimages: extracts raw images from PDF files
* pdffonts: lists fonts used in PDF files
* pdfdetach: extracts attached files from PDF files