From what I can see there, here's what's going on.
Something is editing an XML document and it's using an XML schema to do that. Now, we can't see any of those three things in your post but we can see the result: the schema says that in Line 251 of the document, you've used "application/pdf" as the value of something and you aren't allowed to do that. It looks like the schema contains an approved list of what you can put in that location, and "application/pdf" isn't in that list.
So, you should have a look at the XML document and fix it. Or maybe somebody else created the document and ran it through your application? If that's the case then you would have to send the document back and get them to fix it. Anyway if that doesn't help, maybe you could tell us some more about the background for this question?
I’ve looked at a lot of different solutions, and in my humble opinion Aspose is the way to go. Here’s the link: http://aspose.com