The portable document format abbreviated as “pdf” is the dominant format being very widely used to digitalize the documents. In Windows, Adobe Acrobat Reader can be used to view the pdf files and in Linux applications like Xpdf and a couple of other applications are available for that purpose. But what about editing the pdf documents. Imagine you have received a pdf form to fill in and send back, a paper to review and comment on, or a contract to review, fill out and sign. How all these editing tasks can be done? Apart from adding text to the files for which appropriate form fields are created, it is not possible to use free Adobe Acrobat Reader to edit the pdf documents and you have to pay and buy non-free Adobe Acrobat product in order to be able to manipulate the pdf files. You can also use Adobe Photoshop for that purpose but it is not convenient to edit pdf files with Photoshop as it is with Adobe Acrobat.
In Linux, there are a couple of other (of course free) ways to edit a pdf file. The first way is to use PDFEdit package which is built specifically for this purpose. You can download the package from here free of charge. Here you can see some screenshots of the program. The editing can be done by using GUI and/or scripts. Many of the properties of most object within the pdf files can be edited by this package. For some purposes, I found PDFEdit even more powerful than Adobe Acrobat.
Another less-powerful package for simple editing of pdf files is pdftk. Simple tasks such as splitting and merging of pdf files (and a couple of other tasks) can be done by using this package.
The other way of editing pdf files is to use graphical softwares such as Xfig and flpsed. These softwares are not primarily designed for editing of pdf files but still can be used for pdf editing. See this article about how to edit pdf files in Linux using these two packages.
There are other approaches to pdf editing in Linux. For example, see this writing about how to edit pdf files in Linux. Other methods are out there as well. Give it a Google try!