Hi Richard, I haven't used the MATE desktop environment, but I guess it also has this: 1. Right click on a PDF file, in the pop-up right-click menu, chooss 'open this file with ...' 2. In the then-popped-up dialog asking which program to be used to open this PDF file, select the program desired, then tick on the 'set this as default' checkbox, usually located at the bottom of the dialog.
I've used this method to set default programs, the key is that ticking on the 'set this as default' checkbox. However, since I've not used the MATE desktop environment, there might not be this thing - but it probably has. Besides, in my memory this kind of default-program setting should be handled by xdg-open? Maybe checking 'man xdg-open' would suggest the location of the configuration file. Hope you could find a solution soon, Miriami
