Document management vendors
The majority of the document management providers you'll work with are resellers, not manufacturers. Most manufacturers lack the large in-house tech support and sales teams needed to support a wide customer base. Instead, they support reseller networks who handle much of the front-line interaction with customers. Some manufacturers also sell direct, but they tend to focus on large enterprise level customers.
What to look for in a reseller
The vendor you choose should have experience in your industry. Some industries - legal and medical are the two best examples - have very specific needs from their document management systems, and so require a reseller who understands those needs. Don't depend on the vendor to know everything about your industry, though: you're responsible for compliance issues, not them.
Longevity is another important consideration: you want someone who will be around to support you.
The supplier should also be able to demonstrate more than flashy features and technology, but how those features will help you. Look for suppliers who ask real questions about your business and describe how their product answers your needs.
Watch how long it takes the vendor to respond to you. If they are slow to respond during the sales process, things aren't likely to get better once you've signed a contract.
Size of the company isn't that important - as long as they have the staff to support you, a smaller vendor is fine; in fact you may get more attention than you would at a larger vendor.
What to look for in software manufacturers
Remember to check out the software publisher each reseller represents, as well. Look for proven dedication to the system - make sure the software publisher is committed to updating and improving the software.
In general, look for publishers that specialize in document management. A recent trend in the industry is for makers of specialized vertical applications to add document management modules to their existing software. While this is an attractive option to companies who already use their software, it's not recommended. There are a couple of dangers with this approach.
First, tying document management to your business applications locks you into both solutions at the same time: you'll have trouble getting your documents out of the system if you decide to switch. Also, because document management is just a small component of their main business, it's hard to tell if they're committed to maintaining and improving the system.