MuPDF

is a free and open-source software framework written in C that implements a PDF, XPS, and EPUB parsing and rendering engine. In Gentoo, several packages like or  use it internally for pdf rendering.

Binaries installed by MuPDF are:
 * mupdf-x11
 * mutool
 * mupdf-x11-curl
 * muraster
 * mupdf-gl
 * mujstest
 * mupdf

MuPDF is said to provide optional support for interactive features such as form filling. However, documentation of these features seems well hidden.

External resources

 * http://www.linuxfromscratch.org/blfs/view/cvs/pst/mupdf.html
 * https://packages.debian.org/stretch/mupdf
 * https://wiki.archlinux.org/index.php/MuPDF
 * https://bugs.ghostscript.com/show_bug.cgi?id=697334 - pdf form filling