Whispersync-like experience for PDF or TXT files?

Here are a few options you could check out:

1. Instapaper
   • Acts a lot like Whispersync for any article you find on the web.
   • You can read via a nice iOS app or the web app.
   • You can also forward txt into your Instapaper account via email.
   • It's supported by a ton of additional iOS apps.
   • You can use actually link it right into your Kindle natively through Instapaper or using Ephemera.
   • It does NOT do PDF that I am aware of.
2. Simplenote
   • A slick app to sync text files across devices including your Mac and iOS devices.
   • It has lots of third party support.
   • It does not do PDF or any of the Whispersync-like reading progress syncing.
3. Good Reader + Dropbox
   • You can sync PDFs and other eBook formats easily between your Mac and iOS devices using Dropbox.
   • This solution will NOT do the Whispersync-like reading progress syncing.

As far as I have seen I think Instapaper is going to be your best option for getting a very similar experience to what you're looking for.