This command converts a large .pdf file into a postscript file which is a format that will result in a very large file size compared to the initial .pdf file. The second command takes the postscript file and recompresses it into a .pdf file which should result in a smaller .pdf file. This command is useful when trying to email large .pdf files.
pdf2ps large.pdf very_large.ps
ps2pdf very_large.ps small.pdf