>>> "but he can't change the contents of the file." <<<
...this wholly depends on the end users software really, PDF files can be edited with special acrobat software, and even with acrobat pro on it's own (to a degree)...
...admittedly PDF editing is troublesome for most users, even with the correct software, but it is worth remembering that PDF files are indeed editable, even the so called "secure" pdf files...
...it also depends what you are trying to edit, in theory a PDF 1.3 file is harder to edit than a PDF 1.4 or higher version, due to transparency flattening in PDF 1.3...
...print providers who use Adobes APPE rendering will largely want PDF 1.4 or higher as this allows easier editing very late in a print workflow, even right up to the point the file has been imposed to large press sheets. To fulfil those very tight deadlines in prepress this can often be the difference between delaying a job and chasing around for clients to resend, or fixing it yourself and getting the job done...
andrew