Limits of ZendPdf

Hello,

I'm using ZendPdf since few months and it works pretty well. However
it seems to have some performances limitations when it comes to load
big documents. For instance, I tried to load a 3500 pages doc but it
took more than 30 sec (and I got a fatal error because of time limit).

I'd like to add a stamp in a document, the best case would be on each
page but I well understand that this might be to heavy for a pure PHP
lib, especially on big documents. For me it would be ok to stamp only
odd pages, or 10% of the document, or even  the very first or last
pages of the doc (whatever is easier).

Is there any other ways than ZendPdf::load to get an access of the document ?
Would it be possible to only load and modify the 10 first pages, or
the 10 last or 1 random page?

Best regards,
Manuel