I upgraded to Quickbooks 2004 because they would no longer support the
emailing feature in the older version I was using; to email it
converts the invoice to PDF (you are also supposed to be able to save
the invoice to disk as PDF), but I keep getting an error: "Quickbooks
is unable to access the requested file. It may already be open. Check
the location of the file and try again." After clicking OK it says
"Could not print to printer. Check your printer selection [checked
w/Intuit, according to them it's allright].  Printing may have been
cancelled from another program [I've tried when there are no other
programs open, no difference]." Clicking OK again it says "Quickbooks
was unable to convert your form(s) to PDF. Your form(s) has not been
sent. SOLUTION: Try emailing your form again [doesn't
work]. It may help to close other programs or send forms one at a time
[there are no other programs open and I'm only sending one form
anyway]. If forms do not display correctly in Print Preview [they do],
this could indicate a printing problem [all forms print properly, they
just don't convert to PDF].  If the problem persists, contact Customer
Support [I have and they have not been any help at all]."  I'm running
XP, I've cleaned my HD w/Norton Systemworks repeatedly, have Acrobat
Reader 6.0 installed (I actually unistalled Acrobat 5.0 just in case;
no difference), and worst of all: I installed QB Pro 2004 on my laptop
(I only get to use it 10 times), and there it works just fine!!! 
Help!

I'm having the same problem!
I also have Quickbooks Pro 2004, however it was a fresh install not an upgrade.
I installed the software earlier this year(2004) and about two months
ago I lost the capability to save my documents as PDF's.  Now I can
not email my customers invoices, PO's etc!
I've contacted Quickbooks customer support twice.  They can not verify
that they can solve this problem.  My last call was yesterday.
I'm considering a fresh install, however I would like to know why this
has happend and how to prevent it in the future.

Just encountered same error w/ new install of Nonprofit edn (QB
Premier 2004). Have not found answer; install on second machine in
same office did not generate this error. I have uninstalled and
reinstalled the printer with no change in the error message.
www.amyuni.com is the source of the printer driver, and I have not
checked in with them yet. I will do that, then do a full reinstall,
then see what happens and report back here. Meanwhile if anyone else
gets some news on this please post.