Primary sources are documents that are close (temporally and experientially) to the phenomena under study.
Primary sources may be witnesses or versions of particular texts, memoirs, travel writings, diaries, government documents, works of literature (essays, short stories, etc.), treatises, pamphlets, articles in newspapers / magazines / journals, recordings, films, photographs, etc.
They may be published or unpublished and in any of several different formats (handwritten, printed, photographic, moving image, audio, etc)
Considering a text mining / text analysis project? See our Text and Data Mining Support page and Digital Scholarship research guide for resources on getting started.