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HeinOnline's U.S. Supreme Court Library has PDFs of the United States Reports, beginning with Volume 1 in 1754.
The Supreme Court website provides PDFs of the United States Reports, beginning with Volume 502 in 1991. It also provide PDFs of more recent (un-bound) term opinions.
LLMC Digital provides access to a number of older reporters.
Westlaw also provides PDFs of many of the more recent cases.