This will depend on what kind of historical document it is. Here are some ideas, but it's best to Ask a Librarian or book a research appointment if you have a specific thing you're looking for.
US Government documents: search Google for the title or keyword and add "site:gov" to the end of the search (ex. constitution of the united states site:gov)
Important historical figures: search Google for the person's name in quotes and add "archive" or "collection" or "papers" (ex. Rachel Carson collection).
