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GoodReader - $5.49 (I-M)

The value of this popular and well supported app does depend on your need for a pdf viewer. The added values rom this program is its ability to annotate freehand or as text, highlight the text within the pdf or use "sticky notes". It will also handle large files up to 100mb in size. Whilst it is mainly used for pdf files it does support a variety of other text file types.
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Numbers - $10.49 (M)

The need for a spreadsheet program will determine whether this is an essential app or not. Spreadsheets such as Excel are very highly featured programs - Numbers is more a basic spreadsheet so your opinion of its value will be guided by how proficient you are in Spreadsheet use. If you are a high powered spreadsheet user you will probably be disappointed but if all you require is basic functionality then this will probably suit you. It does support formulas and the usual text and graphics options. Will import and export to Excel and csv formats as well as its own numbers format.
Prices are in $AUS and were correct at time of publication but are subject to change.