When searching UK Parliamentary Papers: it is far easier to identify relevant papers by using an index or checking the references and bibliography in a book on your topic than it is to find material by keyword in this very large digital collection. If you have a Command Paper number (beginning with C., Cd., or Cmd.), you can search by that number.
Besides House of Commons Parliamentary Papers, there was another online index to the parliamentary papers: BOPCRIS (British Official Publications Collaborative Reader Information Service). BOPCRIS covered publications from 1688-1995, and included some full text. It has been subsumed into UK Parliamentary Papers.
Some researchers may be familiar with the CDROM Index of British Parliamentary Papers. Which covers 1801 to 1922 (Lords) and 1944/45 (Commons). It has been superseded by the House of Commons Parliamentary Papers digital collection.
Indexes were regularly published as part of the Parliamentary papers, and those indexes are included in the digital editions.
Some helpful guides and indexes:
For general advice on what is available online, see Parliamentary Papers -- UK Parliament.
House of Commons
House of Lords
For current material, see Parliament's website at http://www.parliament.uk/publications/index.cfm.
A selection of Parliamentary papers have been reprinted by the Irish University Press. There are two sets, the first comprising one thousand volumes and the second two hundred. They are arranged by topic, e.g. Fisheries, Colonies, Civil Disorder; or in the case of the area studies series, by region, e.g. China, West Indies, New Zealand. If there is a set on your topic, it may speed your research. Most are in Firestone, dispersed through the stacks, but some have been sent to RECAP. To find them all, do a keyword search in the Catalog for "Irish University Press" and parliament. There is an 8-volume index to the series at:
House of Commons sessional papers of the eighteenth century: introduction and list. Ed. Sheila Lambert. Wilmington, Del.: Scholarly Resources, 1975-1976. History Reference (SH): Oversize J301.K631q
This accompanies a 145-volume set of reprints at Firestone Library (F) Oversize J301.xK63q, which is replicated in House of Commons Parliamentary Papers.