As far as I understand, they ask you to check the document after conversion to PDF, and not to proofread it.
For such jobs I usually charge per hour. Sometimes I even do this for free - in case they send me my own translation after doing some DTP. It is often enough to take a look at the document to ensure that everything is OK with it.
In case the document has been already proofread, there should be no typos in the PDF (one or two, probably, if overlooked in the source document). But the clients usually need to check if the conversion to PDF format did not result in some encoding problems, if the symbols are seen properly etc.
Of course, everything depends on the document itself (i.e. how large and complicated it is) and on the quality of conversion.
[Edited at 2005-05-07 08:54]
Thanks for your reply, Natalie!
The document is about 20 000 words, so, of course, I wouldn't like to do it completely for free.
I forgot to say that I usually charge per word for proofreading.