Job seeking question: directly contacting a company

In a nutshell, I work for a large company in a fairly niche role. There’s a small firm in my City that specializes in what I do. In checking out their website and LinkedIn, there’s no ‘Career’ section, so any correspondence would go directly to someone.

Is it as simple as sending an email and resume and a ‘hey I’m interested if you’re hiring’ type of email? I have a pretty good resume, and it matches with what I do, and I’d like to be in a smaller firm versus a large corporation (pros/cons aside)

Good idea? Bad idea? Anyone done something similar? Thoughts?