Identify document's owner by content Can anyone suggest a machine learning algorithm that would be useful for identifying the organization a document belongs to? I'm working with a relatively large number of organizations (on the order of 1000s). Naturally, the input data is unstructured, but most documents look almost, but not quite, the same. Also, there are some organizations that follow more then a one template, again, in that "almost but not quite" manner.

Documents are in PDF format, and are reports of some kind and can contain several rectangular regions of text and tables. Most of them are not more than five pages long.

New organizations can eventually appear - how does interfere with algorithm?
Additional to the content of the document, the form could contain even more valuable information. I can't comment on the formatting of pdf files, but it should be possible to exploit the pdf metadata. Author, Title, subject or keywords could provide some clue pointing to a common origin in the same organization.