These databases vary tremendously in scope and quality. [Many have digitized documents from microfilm.] Not all documents, particularly manuscripts, have be transcribed, so don't be disappointed with low search results. You may need to approach some collections as "digital microfilm" and browse through page by page.
Many items are out of copyright given their age, and may be included in multiple databases. If you discover a document in one database, but the image quality is poor, a better copy may be included in another resource.
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