Encontrou isto nos fóruns do Ubuntu :
# gs -sDEVICE=pdfwrite -dCompatibilityLevel=<VERSION> -dPDFSETTINGS=/screen -dNOPAUSE -dQUIET -dBATCH -sOutputFile=output.pdf input.pdf
Só testei isso e funciona bem, vou postar alguma saída agora.
[14:08:41]lee@barlow:~/superuser # pdfinfo org_1-4.pdf
Title: CV 2009
Producer: Prince 7.0 alpha 16 (www.princexml.com)
Tagged: no
Pages: 3
Encrypted: no
Page size: 595 x 842 pts (A4)
File size: 51793 bytes
Optimized: no
PDF version: 1.4
[14:08:59]lee@barlow:~/superuser # gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.5 -dPDFSETTINGS=/screen -dNOPAUSE -dQUIET -dBATCH -sOutputFile=out_1.5.pdf org_1-4.pdf
[14:09:35]lee@barlow:~/superuser # pdfinfo out_1.5.pdf
Producer: GPL Ghostscript 8.64
CreationDate: Wed Aug 19 14:09:34 2009
ModDate: Wed Aug 19 14:09:34 2009
Tagged: no
Pages: 3
Encrypted: no
Page size: 595 x 842 pts (A4)
File size: 60658 bytes
Optimized: no
PDF version: 1.5