For Mac OS X PDF file format is the most important part because the technology behind the graphical interface, Quartz is fully based on PDF.
In order to enable things like that of printing a file to a PDF printer all the operating systems require third party tools while OS X has this feature in it.
There is also lightweight PDF viewer application in OS X called as Preview which allows the image viewer to double up.
Opening the two PDF files and then drag them in a simple Preview so that both the files are viewed on the same page. It also helps in changing the order of the pages depending on what document you drag first on the page. The best thing about PDF is that it can easily combine a lot of files by opening them using the Preview.
The PDF allows you to select the pages that you want to send to your friend by dragging them on the separate folder or on Desktop by creating a separate PDF file of selected pages.
You can easily delete pages from the PDF file that are not for your use. The advantage of deleting pages is to short the eBooks and read only the relevant material.