You can too simply try to change the extension of the file into jpg to be able to attach it here (make a backup of the original of course ;)).
If you have for example the file "manual.pdf", change the extension this way : "manual.jpg" (it should say something "are you sure to do this ?" accept, since you've made a copy of your original file). Now you should be able to upload it attached with one of your forum message since it's considered to be a picture. After that, we only have to download the file, and to revert the change : change the extension .jpg to .pdf : in theory, we should have the file (but it may not work :P).
The problem with your suggestion is that the maximum size gog accepts in its attachments is 2Mbs (which is smaller than the 8mbs the pdf he has is). You can see it if you attempt to make a new post, it says, under "attached images", "0 of 2MB used". So that's off sadly, it'd be more convenient for everyone if gog's attachment limit was bigger...
Edit : And you can of course contact GOG staff to send it to them too :)
If this doesn't work or you simply want to directly send us the pdf via email (without uploading it into a host like dropbox/mediafire etc...), tell it to me or Treasure, we'll see what to do :)
I don't know about sending stuff to gog via email. Afaik, some people attempted to send pdfs of their manuals, sometimes they get added, other times not, most probably because the manual wasn't included in the negotiation with the developer/publisher - so it's a hit or miss situation, and it's better if he does both (that is give it both to us and to the gog staff) so that all bases are covered. But ofc he doesn't have to send it to gog, as this might not bear any results...
Hopefully, he won't have a problem with using a host, as that's both the easiest and most private way for him to upload and for us to get access to the pdf that I can think of (especially when using Google Drive as it doesn't show the username of the uploader, contrarily to Dropbox -don't recall the situation with OneDrive...)