Evince 

Evince is a document viewer for multiple document formats. It currently supports pdf, postscript, djvu, tiff and dvi. The goal of evince is to replace the multiple document viewers that exist on the GNOME Desktop with a single simple application.

Main features:

- Integrated search displaying the number of results found and highlights the results on the page.

- Thumbnails of pages show quick reference for where you'd like to go in a document.

- When index information is included in a PDF Evince will display it in a tree format.

- Using the GNOME/GTK printing framework Evince can print any document it can open.

Evince can open PDF documents that have been encrypted

- Evince implements ATK interface, so it's accessible. To know how to use or how it's implemented.