There are subsets of Baraha fonts (BRH Devanagari Extra) embedded in the PDF. Luckily, these are free fonts. If you'd had embedded subsets that are proprietary fonts, there wouldn't have been much you could have done.
Just search for these fonts and embed them using Sigil and you should be good to go (assuming, as Idolse wrote, that Sigil is cimpliant enough to use embedded fonts). I think it's worth a shot. If you have trouble finding the fonts separately, just install Baraha (it's a free Devanagari wordprocessor that I've used for Marathi in the past
). These fonts come with it.
Note: just to be extra careful, open the pdf on your PC (or Mac
) and go to its properties (the fonts tab). Make a note of all the fonts used that are not standard (as far as I could tell, it's just the one I mentioned above). Download them all and embed them.