A helpful site for church documents is Papal Encyclicals Online. You may browse by Church Council or by Pope. There is also a search engine, pictures of medieval Papal documents, etc.
"Denzinger" is a sourcebook of Catholic dogma comprised of the creeds of the faith, beginning with the twelve apostles, and documents of the Church's Magisterium, through the pontificate of Benedict XVI. It is arranged chronologically and includes papal bulls, encyclicals, conciliar decisions, doctrinal decrees, etc.
If the document was issued after 1878, consult the Papal archive on the Vatican Website. Note that many of the older materials may not be available in English.
If the document was issued after 1930 consult the Catholic Periodical Literature Index (in print) for 1930-2004 or the ATLA Religion Database for 1981-present. Within ATLA, go to Advanced Search and under genre, choose "papal document."
For documents before 1740, consult Papal Registers. For Papal Bulls (in Latin) from the time of Leo the Great to Benedict XIV, see Bullarium Romanum, available in our Special Collections. The English version for some older papal bulls may be found on Papal Encyclicals Online.
The Latin text represents the most authoritative version of a given document, with some exceptions; therefore, the Latin source is generally preferred for purposes of citation. Certain significant Papal documents may be published in English as standalone publications. Other sources may contain English translations of Latin originals. These include the periodicals The Pope Speaks (1954-2005), L'Osservatore Romano [English Edition] (1968-present), and Origins (1971-present).
Another way to search the Maly Catalog is via subject headings:
Papal addresses and documents fall into certain recognized categories with levels of authority relative to each other. Below is a description of the major types of Papal documents, with those of greater solemnity near the top of the list. More details on some of these documents may be found in the New Catholic Encyclopedia.
The information for this guide was taken from Helen Hull Hitchcock's article on the authority of church documents and Jimmy Akin's book Teaching with Authority: How to Cut Through Doctrinal Confusion & Understand What the Church Really Says.