Both words are correct and have the same meaning. However, it is more common to use "imprimido" (regular form) in the present perfect tense: "Yo he imprimido los documentos" (I have printed the documents); whereas "impreso" (irregular form) is usually used as an adjective: "Por favor, trae los documentos impresos" (Please, bring the printed documents). Another example of two possible participles in Spanish is "frito" and "freído", that come from the verb "freír" (fry -> fried)