I've been using the [Mendeley Desktop][1] application for a couple of years now.

#Features

It meets all of your requirements. Once you've imported bibliographic data via the Windows desktop app, you can also see it via the web interface.

###Attach the actual documents
Each reference can have zero, one or more documents attached to it. These are full-text indexed, and searched. You can do some advanced searching, for example looking for specific text in an author's name, or in the title. There's a DOI lookup to pull data in from web databases, and it will scan a PDF for metadata and incorporate that into the database. It's reasonably good, given the huge range of formats that people use to specify a paper's metadata.

###Organise the citation
It imports the authors' keywords. It also allows you to specify your own tags, in a different field, so these are kept separate from authors' keywords. You can also assign papers to groups, in a many-to-many mapping. It has a flag for read/unread. You can use groups to set reading priority: create groups called "must read", "read in 2014", whatever, and add files to groups as appropriate.

###Import/export

You can import & export biblio data with other applications (RefMan, EndNote, etc):

- Bibtex .bib file
- EndNote .xml file
- Research Information Systems .ris file

You can also import from:

- Zotero .sqlite file

And you can export to MS Word in any one of dozens of citation formats. There's a Word macro file which manages this, and which allows you to change the citation format once imported.

#Caveats

- In the early beta days, it did corrupt some metadata, but that seems to have settled down now.
- Mendeley HQ aren't very good at support or feature requests.
- It is a *gratis* (zero-price) product, and you know what they say about those: if it's free, you're not the customer, you're the product.
- Note that just as with any other reference management software, using the Word macro may cause issues when you edit a Word document co-operatively with other people, when they don't have the same reference database as you.

#See also
[Related questions on the Academia StackExchange][2]


  [1]: http://www.mendeley.com/download-mendeley-desktop
  [2]: http://academia.stackexchange.com/questions/tagged/reference-managers