Whispersync-like experience for PDF or TXT files?
Here are a few options you could check out:


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.

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.

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.