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Just to add on Wallcraft's comments PDF stands for Portable Document File, it is the predecessor of PosctScript. It's original creation was to act as a container file for Postscript but has evolved over the years (from a tech view not a marketing view) to support images, security and much more.
The design of PS/PDF has always been on how to layout the document. (Did you know: PS/PDF was never designed for printing it was an interpretive language designed for displaying text on different devices, (e.g. screens, terminals, printers)).
The latter part is why we get in trouble with PDF. Because it has been compiled to work on a set screen size. If PDF was optimized for 6" screens you would have the best reading experience.
However most PDF are designed for 8x11" paper or books that are 9" wide. In addition most PDF makers add 1/2" margin and page numbers. When a PDF is rendered it renders one page at a time to screen. Since the page is rendered like an image this is why the fonts and images are scaled up or down in order to fit the screen size.
For 5"/6" eInk devices this means a 8x11" inch page is getting squeezed down to 6" making reading a PDF next to impossible.
I often remove the white margins and the page numbering and the PDF tend to look just right for me.
Ahi has made some really nice PDF for 6" device, also FeedBook does too.
As I mentioned earlier a PDF is really a container file, it can included images and text. Because PDF is so widely supported many folks create an imaged based PDF and distribute them as PDF files. You'll know when you have an imaged based PDF simply by the size. Also no search or highlight capability will be present.
Don't associate OCR with PDF they are separate beasts and do not have any connection. PDF do not have any built in OCR capabilities.
Typically an OCR software will convert a text base image to PDF but PDF is not the wiser.
Last point on reflow. As wallcraft mentioned Image based PDF will not reflow, however an text base and image base will reflow, typically placing the image in the right spot and reflowing the text around the image.
Right now most eInk reflow is done by Adobe DE SDK and it is terrible. Images are dropped and the text looks blocky.
There are some software out there that do reflow PDF and they look fantastic. Adobe Reader is one of them, there is a company called RepliGo that also refows PDF very nicely
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