Archives traditionally include collections made up of physical items: personal papers, unpublished manuscripts, photographs, correspondence, and so much more. Digital preservation of archival documents is now underway at many repositories, and accelerating every day, making remote access possible. However, the vast majority of archival materials are still not yet digitized, which is important to keep in mind as you search online.
We have a thorough and detailed research guide on archival research. Below are links to parts of the guide that are specific to online research with primary sources, including many links to digital collections that may provide a place to start.
Getting Started | Analog vs Digital | Archival Search Tools | Government Archives | Highlights | International Resources | Library Databases | Online Portals | Papers Projects | Regional Collaborations | Research Libraries - Digital Collections | Subject Collaborations | Subject-Specific Libraries & Archives | Texts, Images, Maps, A/V - more links | Volunteer Crowdsourcing Projects | Why Digitize This But Not That?