I use PDF SAM all the time (http://www.pdfsam.org/). It's an easy utility to use for splitting or merging PDFs. I use it to create an expense report PDF, for example, bringing together invoices and statements from a bunch of different sources. Other than digital signing (which, like @Nino, I haven't found in an open source viewer yet), it's the thing I do most often with PDFs.
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I use PDF SAM all the time (http://www.pdfsam.org/). It's an easy utility to use for splitting or merging PDFs. I use it to create an expense report PDF, for example, bringing together invoices and statements from a bunch of different sources. Other than digital signing (which, like @Nino, I haven't found in an open source viewer yet), it's the thing I do most often with PDFs.