Thanks! I used Inkscape to open the pdf file - the part's outlines are editable as vectors then (with more or less clean-up work needed).
Subdivision surfaces are useful for this kind of smoothing of low-poly model, but the model must be constructed with that in mind - the topology would have to be changed to use it on the model as a whole. But with some tweaks I managed to get some nice smoothing on a few areas. The downside is, that the UV layout got messed up and it would probably be more difficult to unfold (at least in Blender: co-planar neighboring faces confuse the unfolder, but there's a tweak for that).
