If you double-tap in the middle of the page, it will be enlarged to remove the grey margins.
Good Reader is a bit expensive but it's worth it IMO, mostly for the great file management functionalities. It's also useful to annotate PDF files, for example if you want to fill a form, and you can crop the margins of a PDF file which are often too large for comfortable reading in the iPad.
But if you are looking for free apps, I suggest you try searching for PDF in the iTunes store, I'm sure you will find a lot of them.
"The thing about witchcraft," said Mistress Weatherwax, "is that it's not like school at all. First you get the test, and then afterward you spend years findin' out how you passed it. It's a bit like life in that respect."
Terry Pratchett (The Wee Free Men)
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