If you are pitching your idea or services to multiple parties, the one thing you do not want to do is send out a general form letter along with a standard brochure or stack of resumes. That sort of 'one size fits all' approach cannot substitute for a real proposal. The goal of a proposal is to persuade the client or boss to endorse your idea and let you do the job. To succeed at convincing them, you need to focus your message to a specific situation, gain their trust and show them that you know what you're talking about and can deliver what they need.
In proposal writing, your first step should always be to gather information about the party who will judge your proposal. That's because you want to present a proposal tailored to that party's specific needs and knowledge level. In other words, you need to put yourself in the other party's shoes and look at the situation from that party's point of view. If you are pitching to your boss or your company executives, you might already understand their positions and their concerns. But if you are pitching to people at another company, then you will need to do a bit of work researching who they are, what they do, and what their needs are. Yes, that research can take a bit of effort, but putting in that effort makes your proposal much more likely to succeed.