Reading PDF through FPGA
First of all you need to determine what you're going to do with the PDF. If you're displaying it, you'll need some memory for the frame buffer, and a way to interface with the display. That should be your primary concern. Then, start thinking about rendering the PDF itself. As PDFs are essentially compressed PostScript of one form or another with embedded fonts, you can divide your tasks into some major elements:


part of the FPGA to load data from SD card or other media including file system access
part of the FPGA to decompress chunks of data, or to accelerate this process
part of the FPGA to decompress image data (JPEG, PNG, etc.) and copy to memory
part of the FPGA to decode/execute PostScript
part of the FPGA to figure out what needs to be displayed (such as the current page); this could be simple bounding logic or complex scaling logic handling a variety of different display modes
a floating point unit for the floating point coordinates in PostScript
an integer ALU
several "GPU" engines which render primitives from the PostScript engine (ideally, in parallel) e.g. draw line, draw polygon, ...
a font engine or two to render fonts (this will likely be very complex as you will need to support complex features like hinting and antialiasing.)
a display interface and memory interface
a UI controller of some kind, perhaps implementing copy/paste, selections, menus, etc.


Ideally the engines 1-5 would be pipelined to get maximum throughput. You'll be looking at a big FPGA to do all of this.

You could probably do this on a CPU, but if you realllly want to do it on an FPGA, this is probably the route to take.