Conversion between Postscript and PDF

How do I convert files between Postscript (*.ps) and PDF (*.pdf) formats?

PDF to PS conversion and converting A4 page size to U.S. Letter:

1. acroread -toPostScript file.pdf (command line convert pdf to ps)
2. acroread -toPostScript -size letter file.pdf (command line convert pdf to ps and FORCE the output paper size to letter)
3. psresize -pletter orig-ps-file.ps new-ps-file.ps (forces output to letter sized)

PS to PDF:

- or -

1. /usr/local/bin/convert myfile.ps myfile.pdf
   /usr/local/bin/convert -help for more options and info
2. /usr/local/bin/distill myfile.ps
   /usr/local/bin/distill -help for more options and info