Is that market worth entering, especially for selling to small companies? To build chipper document management systems?
Whether or not it is a good market/business is relative to your ability and your alternatives. Why don't you tell us why it is a good idea for you? Based on you knowledge and ability to "sell it", you could get better feedback.
Well, there is a lot of Open Source Software available as it is with other topics. However, if you provide features which people like to have, then way not. Solve the problems of small companies about document management, that's the focus you need.
It's always the same, you will not really find a market these days, which hasn't some solutions or products to offer. You are lucky if you find such a market.
Find out what others are offering and do one of two things, make it better or make it cheaper. Or you are able to do both, being better and cheaper. But keep in mind, cheapness is not always a good way to market.