very slow pdf download from 1 client, but not others

i've got a client who's getting very slow downloads of PDFs on my
site. i've tested my server from several other machines and gotten
100+ kbps dowload speeds by right clicking the PDF and choosing "Save
targer as...". If I do the same thing from my client's machine,
they're downloading at 5 kbps. If i download PDFs from other servers
on my client's machine, they're downloading at 100+ kbps. It seems to
be a networking issue, but what? We're both using the same ISP, but
have totally different networks otherwise.

any ideas?

tks