I want using any free Linux document viewer that does both pdf and djvu (Okular, Evince, etc.) to do the following: magic_command('document-viewer','./11.pdf','instance1') magic_command('document-viewer','./21.djvu','instance2') two instances (windows) of document-viewer are opened one with 11.pdf and the other with 21.djvu then: magic_command('document-viewer','./12.djvu','instance1') magic_command('document-viewer','./22.pdf','instance2') the first instance (window) of document-viewer loads 12.djvu the second instance (window) of document-viewer loads 22.pdf