Bug 6679 - [SIGNED-OFF] fix 4 perlcritic violations in C4/Creators/PDF.pm

Bug 6679 - [SIGNED-OFF] fix 4 perlcritic violations in C4/Creators/PDF.pm

- "return" statement with explicit "undef" at line 171, column 8.  See page 199 of PBP.  (Severity: 5)

- "return" statement with explicit "undef" at line 201, column 8.  See page 199 of PBP.  (Severity: 5)

- Bareword file handle opened at line 212, column 11.  See pages 202,204 of PBP.  (Severity: 5)

- Two-argument "open" used at line 212, column 11.  See page 207 of PBP.  (Severity: 5)

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>