incompatible PDF format
If you go to http://jasss.soc.surrey.ac.uk/13/1/9.html and look at the article, you have the option of saving it as a PDF on your machine. The PDF looks OK on a Mac, it has the right icon and it opens fine with Acrobat or Preview. Zotero however does not recognize it as a PDF and will not even give you the opportunity to look for metadata. This problem shows up in anything you generate from that website. The workaround is to open the file in Acrobat first and then do a save as to another name. This file comes out a slightly different size and Zotero handles it just fine.
http://www.zotero.org/support/dev/creating_translators_for_sites
Also you can find the pdfs for the articles on that site by deleting the '.html' and then adding '/number.pdf' where the number is the same as the one before the '.html'. For example, for the article at:
http://jasss.soc.surrey.ac.uk/13/1/9.html
the pdf can be found here:
http://jasss.soc.surrey.ac.uk/13/1/9/9.pdf
Select the icon to the left of the address bar in Firefox and you can drag it straight into Zotero.
In that case, I can't reproduce your problem. When I try, Zotero recognises the file without any problem whether I download it from the website and then drag the file into Zotero or whether I just drag the direct link I gave above directly into Zotero.
I am using Windows Vista.