When keyword searching for primary sources, remember that the term "World War I" was not in use until World War II. Even the term "Great War" probably was not used from the start of the conflict. The terms used to describe this conflict changed over time and will be different depending on the author of the source. Consider searching for specific battles, geographic locations. using terms like "European War" to find materials.
Searching "World War I" in the library catalog or any database based on the Library of Congress subject headings will not retrieve all materials. The correct subject heading for World War I is "World War, 1914-1918" which is what should be used to search for materials related to World War I. Keyword search "World War, 1914-1918" and then add the additional keyword that indicates the kind of primary source desire.Examples are correspondence, letters, personal narratives, documents, and sources.
Here is a link to the KU catalog for personal narratives (a term for memoirs, diaries, journals) from World War I.