Microfilming, digitizing and preservation photocopying of collections are often done both to preserve the originals and to improve access to the materials, which might be rare and/or in fragile condition.
When a particular item is available in a surrogate format, researchers will be given the surrogate to preserve the original.
Primary sources originally in manuscript form, such as correspondence, diaries, ledgers, maps, and other hand-written documents, may be availabe in printed and/or electronic editions. The same is true with speeches and broadcasts, for which transcripts may have been published.
One example of printed correspondence is The Papers of George Washington (Washington, George, and Dorothy Twohig. 1998. The papers of George Washington. Retirement series. Charlottesville: University Press of Virginia.)
Find printed editions of an individual's manuscripts in BobCat or WorldCat with a search by Author or Subject. Subject searches may take the form "Author -- Correspondence" such as "Abraham Lincoln -- correspondence" or "Presidents -- United States -- Correspondence."
One fine example of manuscripts published online is The Mark Twain Project Online, a website whose, "ultimate purpose is to produce a digital critical edition, fully annotated, of everything Mark Twain wrote. "
There are advantages to using surrogate formats: Microfilm and books are often available for interlibrary loan. Printing from microfilm for reference use is easy and inexpensive. And digitized primary source materials are becoming more widely available every day.