The MuPDF manual has a new page- http://mupdf.com/docs/manual
However the short-cut keys are not intuitive (traditional keys like Home
do not work), cannot be re-mapped and are not well-documented (I had to guess that g
opens first page- it is only documented that G
opens last page) but I cannot find a key for opening of a new document without exiting MuPDF.
Besides PDF files MuPDF can also open EPUB, CBZ and XPS files.
Usually I appreciate bare bones programs but this is one is not to my liking.
MuPDF is licensed under Affero GNU General Public License.