Close to the left of your PDA's EQ, there is an 1/8" Line In and (slightly further over) a 1/4" Line In.
The unit's manual is not very comprehensive and doesn't mention whether either are a stereo input and there are no controls (that I can see) for their levels, balance, etc, so: no indication if they are stereo inputs.
If the 1/4" Line In is a stereo outlet*: make or buy a two 1/4" Mono to one 1/4" stereo plug cable and connect the Behringer's 1/4" Outs to that Line In.
*If not: most 1/8" Line Ins are stereo, to facilitate the playing of portable tapes, CDs, etc and you could follow the above idea, but with an 1/8" stereo plug or 1/4" to 1/8" stereo plug adapter.
If neither of the PDA's Line Ins are stereo: go from the Behringer's L/R 1/4" Outputs to both the 1/4" and 1/8" Line Ins with mono cables.
Mickey Mouse, but: it should get you there.