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One of our members carried out a survey of Office apps for his company; you can see his conclusions in this thread...
http://www.ipadforums.net/new-member-introductions-site-assistance/22431-hi-all.html
Another comparison was undertaken here....
http://www.ipadforums.net/iwork-forum/36629-office-app-comparison.html
The Apple iWorks Office suite for the iPad has a set of useful Help pages that show their capabilities here...
Pages is Word compatible and can read .doc and .docx files and save in .doc and .pdf
Keynote is PowerPoint compatible and can read .ppt and .pptx files and save in .ppt and .pdf
Numbers is Excel compatible and can read .xls and .xlsx files and save in .xls and .pdf
Apple - Support - iOS Apps - Welcome
Tim