You might find this script or the commands in it handy if you have
ghostscript.  A4 paper is longer than letter size, and ghostscript
seems to make good decisions about what to chop off if the matter
is too long when invoked in this manner.  You might have to
print the file one page at a time using ghostview if any pages
had to be shortened vertically.

Do not despair if you find something too long.  psresize
will work, but the result is not quite as good.

#!/bin/bash
# (c)2002 David Raleigh Arnold.  Free under GNU.
#
# Usage: pdfa4letter NameOfPDFfile (without .pdf extension).
#
# -- This script converts a pdf file
# formatted for a4 paper to letter paper without
# resizing.  The wisdom of ``ghostscript'' is
# applied to it by using it to convert to
# postscript format, and the postscript version
# is retained as an added bonus.  :-)  It is
# necessary that the a4 original have sufficient
# vertical margin.  This is far from fast, but
# I got a good result.  DaveA
# dra@openguitar.com  http://www.openguitar.com

echo -n "Making Postscript file... "
pdf2ps -sPAPERSIZE=letter -dFIXEDMEDIA $1.pdf $1.ps
echo -n " Making PDF file... "
ps2pdf $1.ps $1-let.pdf
echo "Done..."