There are two popular kinds of document/spreadsheet type programs for PDA's right now, and one of them is Documents To Go Professional by Dataviz (
http://www.dataviz.com). The other is QuickOffice, but it doesn't have the customization options that DTG has... so if you like to fiddle with the documents and spreadsheets, then go for DTG.
Only thing to consider otherwise is price. DTG is more expensive that QuickOffice, but for some... well, price isn't an option

Also check and make sure that your device won't need any special fonts to run either program, as some sony handhelds require them to properly view the program.
Cheers!