The link to the pdf (the href-attribute in the <pdf> tag) needs to be absolute and not relative.
I.E. "document.pdf" will give you the message you mentioned while "http://stuff.com/document.pdf" will work fine.
I know it sucks but that's the solution.
I have an absolute (local) path in the <pdf> tag, for example C:\path\to\my\file.pdf
I'm having the exact same problem. I actually called Adobe and tried to setup a paid support ticket and they wouldn't take my money. They said it's not a problem that they would support. I was trying to get it to work over 2 different network segments but still, it doesn't even work when everything is local.
I've fround a suitable answer. It appears as though the XDP file opens Adobe Reader first and then opens the PDF in a browser window because it's looking to the website for the address (http://www.domain.com/file.pdf).
The answer is to make XDP files open with Internet Explorer, or any other browser I would imagine. Once the extension is told to open with IE then everything works correctly.
Check this page for an explanation:
Adobe Acrobat PDF Reader (Free) X (10) version is working for us in MAC OSX 10.6 Safari!
There was\must be a bug in version 8 and 9 beside being only 32-bit compliant!