Can I get achievements by demo games on steam?

There might be achievements for certain game demos, but the demos are probably considered to be different entities from the Rewards games and not grant you credit if you don't get the full versions of the games as well.

no

Demos have different IDs to the actual games (as far as I know). Civ 5 for example is 8390, but its demo is 65900.

Thanks for the comments, guys. I found Sonic Generations Demo and unluckily there aren't achievements as you said.