MuPDF is an open source software, which can open files with PDF, XPS formats. This software also supports image formats like JPEG, TIFF, PNG too. The User interface is really clean and with out any toolbars.
Supports PDF 1.7 with transparency, encryption, hyperlinks, annotations, searching etc.
Reads XPS and OpenXPS
Can open Image files
Easy to navigate
If you want to compile this application for your android, you can do that too. MuPDF is available for both Linux and Windows.
Are you looking for a clean portable PDF, XPS viewer, you can try MuPDF.
[Via I Love Free Software]