Earth’s atmosphere is mostly made of nitrogen and oxygen, 78% and 21% respectively, with the remaining percentage divided among Argon, traces of other noble gases and carbon dioxide.
The composition of Pandora’s atmosphere is: nitrogen (50%), oxygen (25%), carbon dioxide (18%), xenon (5.5%), hydrogen sulphide (1%), methane and ammonia (0.5%).
A very different mixture compared to the Earth’s atmosphere, and also 20% denser, due to the high percentage of xenon, a heavy and inert gas.
Nitrogen is an odorless gas, as are oxygen, carbon dioxide, xenon and methane, so the smell of Pandora’s air would be entirely determined by that of ammonia (strong and pungent) and of hydrogen sulfide (similar to rotten eggs).
However, ammonia represents less than 0.5%, and may not be detectable, or in any case be less noticeable than the hydrogen sulphide that makes up 1% of Pandora’s air.
It is plausible that a breath of Pandora’s air wouldn’t smell too differently from earth’s air, except for a faint scent of rotten eggs and perhaps an even milder scent of bleach.
The real problem though, would not be the smell of these substances, however unpleasant, but the effect they would have on the body of a human who inhales them.
The human body tolerates an oxygen concentration ranging from 19.5% to 23.5%. On Pandora, oxygen is slightly above (25%) the maximum limit. It would still be breathable but not without reversible damage after short exposure, and possible death after long exposure.
The oxygen level on Pandora is therefore at a concentration between that of today’s Earth and that of Earth during the Carboniferous period (about 30%). This could also explain why the animals on Pandora are much larger than those on Earth, along with the lesser gravity, obviously.
Carbon dioxide, that makes up just 0.04% of earth’s atmosphere, on Pandora reaches a very high 18%. Almost five times higher than the toxic dose for humans, which is around 4%.
That could also explain some features of Pandora. Since it is the moon of a gas giant, it’s likely to be outside the habitable zone of its solar system. Thus, it is possible that tidal forces and the immense greenhouse effect due to that 18% of CO2, warm Pandora so much that it not only can support life, but also has rainforests on most of its surface.
Returning to the atmospheric composition, the icing on the cake is hydrogen sulphide: a real poison, which causes irritation, cough, tachycardia and inhibition of mitochondrial respiration even at doses below 1000 ppm, or concentrations below 0.1%. On Pandora the concentration is at least 10 times higher, and would cause immediate collapse with suffocation after a single breath.
Finally, even if methane is not lethal at the low concentration on Pandora, ammonia becomes so at 500 ppm, a level just slightly higher than that of the alien atmosphere.
In conclusion, a (last) breath on Pandora would slightly smell like rotten eggs and bleach, and it would be lethal especially because of the large amount of carbon dioxide and the mere presence of hydrogen sulphide.