Please don't add the category [[Category:World War I]] or [[Category:United States Army, World War I]] to a profile if all you have to support it is a WWI draft registration document. All of the men filed these, but they did not all serve. National Archives says,
It is important to note that not all of the men who registered for the draft actually served in the military and not all men who served in the military registered for the draft. Moreover these are not military service records. They end when an individual reports to the army training camp. They contain no information about an individual's military service."
You should post other supporting documents: photos, records, news articles, etc. in addition to this draft registration, in order to include someone in the category. You can use the draft registration for other means, such as corroborating a birth date, place, or names of relatives, description, etc, and it is certainly a useful doument. It just doesn't prove that that particular man served in the war.
National Archives has some helpful information :https://www.archives.gov/publications/prologue/1998/fall/military-service-in-world-war-one.html