That suggests either a very like-minded company (with 25 people very likely, with larger sizes less so) or tremendous amount of peer pressure. After all, if out of 25 people 24 decide to share, and 25th wants to work in this company, they don't really have a choice, do they? That person has to work with all these people after it.

These people may not have access to the document, but surely in a small company they would figure out some of the salaries either through the grapevine or shoulder surfing?

So there was the implicit threat that if you didn't join, you wouldn't be eligible for raises? Also, how could you enforce them not seeing the list? It's illegal for an employer to deter the discussion of compensation amongst employees.