If words are highlighted within any program whilst stardict is open, and stardict has the correct option set, it automatically interprets the word and makes a popup with the definition.
The OP is asking how to highlight words in a pdf reader.

Unfortunately, the answer is I don't think it's possible with evince for maemo at the moment - Evince can definitely select text (e.g. select all), but any movement of the pointer is automatically assumed to be a panning request, rather than text highlight.

As for xarchiver, no, that's a frontend for archiving commands e.g. tar zip.