Amazon has made available new Send to Kindle desktop Windows software. It works a lot like Instapaper, but can send any file found via your Windows Explorer.
PDF features in the banner (above), but I’ve not yet seen how badly it mangles PDFs on conversion. Possibly Amazon may not try to covert at all, but rather just send PDFs ‘as found’?
You also get a ‘Send to Kindle…’ option in all “Print” dialogues, including that of your Web browser.