@papukaija:
"- you aren't using the latest Ubuntu. Upgrade it now."

FIRST: I'm using the latest Ubuntu version: 

~$ cat /etc/debian_version 
squeeze/sid

~$ lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 10.04 LTS
Release:	10.04
Codename:	lucid

"- you haven't noticed that this bug is actually fixed."

SECOND: I have the latest CUPS packages.

ii  cups-bsd                              1.4.3-1ubuntu1.2                                Common UNIX Printing System(tm) - BSD comman
ii  cups-client                           1.4.3-1ubuntu1.2                                Common UNIX Printing System(tm) - client pro
ii  cups-common                           1.4.3-1ubuntu1.2                                Common UNIX Printing System(tm) - common fil
ii  cups-driver-gutenprint                5.2.5-0ubuntu1                                  printer drivers for CUPS
ii  cups-pdf                              2.5.0-12                                        PDF printer for CUPS

.... And is not fixed yet !!!!


~$ tail /var/log/cups/cups-pdf_log 
Mon Jun 28 12:24:51 2010  [ERROR] failed to create directory (/home/user/PDF)
Mon Jun 28 12:24:51 2010  [ERROR] failed to create user output directory (/home/user/PDF)
Mon Jun 28 12:28:19 2010  [ERROR] failed to create directory (/home/user/PDF)
Mon Jun 28 12:28:19 2010  [ERROR] failed to create user output directory (/home/user/PDF)
Mon Jun 28 12:29:35 2010  [ERROR] failed to set file mode for PDF file (non fatal) (/home/user/PDF/_ubuntu___SOLVED__pdf-cups_failed_-_Ubuntu_Forums.pdf)
Mon Jun 28 12:31:16 2010  [ERROR] failed to set file mode for PDF file (non fatal) (/home/user/PDF/_ubuntu___SOLVED__pdf-cups_failed_-_Ubuntu_Forums.pdf)
Mon Jun 28 12:40:57 2010  [ERROR] failed to set file mode for PDF file (non fatal) (/home/user/PDF/about_blank.pdf)

~$ tail /var/log/messages
Jun 28 12:28:19 mymachine kernel: [323246.032824] type=1503 audit(1277746099.230:44):  operation="mkdir" pid=29283 parent=29073 profile="/usr/lib/cups/backend/cups-pdf" requested_mask="c::" denied_mask="c::" fsuid=0 ouid=0 name="/home/user/PDF/"
Jun 28 12:29:35 mymachine kernel: [323322.700690] type=1503 audit(1277746175.899:45):  operation="open" pid=29367 parent=29366 profile="/usr/lib/cups/backend/cups-pdf" requested_mask="::r" denied_mask="::r" fsuid=10000 ouid=0 name="/proc/filesystems"
Jun 28 12:29:35 mymachine kernel: [323322.701763] type=1503 audit(1277746175.899:46):  operation="open" pid=29367 parent=29366 profile="/usr/lib/cups/backend/cups-pdf" requested_mask="::r" denied_mask="::r" fsuid=10000 ouid=0 name="/proc/filesystems"
Jun 28 12:29:35 mymachine kernel: [323322.701844] type=1503 audit(1277746175.899:47):  operation="mknod" pid=29367 parent=29366 profile="/usr/lib/cups/backend/cups-pdf" requested_mask="c::" denied_mask="c::" fsuid=10000 ouid=10000 name="/home/user/PDF/_ubuntu___SOLVED__pdf-cups_failed_-_Ubuntu_Forums.pdf"
Jun 28 12:31:16 mymachine kernel: [323423.301164] type=1503 audit(1277746276.499:48):  operation="open" pid=29482 parent=29481 profile="/usr/lib/cups/backend/cups-pdf" requested_mask="::r" denied_mask="::r" fsuid=10000 ouid=0 name="/proc/filesystems"
Jun 28 12:31:16 mymachine kernel: [323423.302197] type=1503 audit(1277746276.499:49):  operation="open" pid=29482 parent=29481 profile="/usr/lib/cups/backend/cups-pdf" requested_mask="::r" denied_mask="::r" fsuid=10000 ouid=0 name="/proc/filesystems"
Jun 28 12:31:16 mymachine kernel: [323423.302276] type=1503 audit(1277746276.499:50):  operation="mknod" pid=29482 parent=29481 profile="/usr/lib/cups/backend/cups-pdf" requested_mask="c::" denied_mask="c::" fsuid=10000 ouid=10000 name="/home/user/PDF/_ubuntu___SOLVED__pdf-cups_failed_-_Ubuntu_Forums.pdf"


"- you haven't noticed that cups-pdf isn't installed by default and that this package comes from the universe repository so it doesn't need to be tested so much."

THIRD: The universe mainainer may  had printed at least ONE FILE and given account of a simple error... It sound like "build and release" like the RedHat way. Ubuntu proudly presents their OS as a Debian based system but the truth is that it's very distant in matter of stability... That's not a particular error, it's present on every system.

"but your comment just shows a lack of knowledge" ... my english is bad ... sorry. But that's not matter of knowledge, it's works in debian, in fedora, in SuSE ... but not in Ubuntu, What amount of knowledge do you have to have to view the facts. And admit that it's not a single/particular error ... and it's not solved!!!