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To be quite honest, things can get tricky. If the document was created with proper form fields, you can add text to the document with the free Adobe Acrobat and print it. However, you can't save your changes. Also, this only works in the official Acrobat program - Xpdf doesn't support PDF forms. Unfortunately Acrobat Professional ($449) is only available for Windows and Mac so that doesn't help the loyal Linux user.
Recently, I was that Linux user. After doing some investigation and following a few tips from others, I found two software solutions. Although these tools are not the easiest to use, they do fulfill the goal of filling out PDF forms. In addition: they point towards what I believe could be a true Linux "killer app": a robust PDF markup tool.
The tools I found are Xfig and flpsed. Xfig is a general image editor, while flpsed is specifically for marking up PDF files. It's important to note that neither of these programs can actually edit PDF files. Rather, they can be used to add new text and graphics on top of the file. This is generally all you need for filling out typical business forms such as contracts.
I do have to warn you: flpsed is not a finished program. The flpsed home page warns: "flpsed is still alpha software!" Having said that, I did find it usable, although a bit rough around the edges.
Given flpsed's unfinished nature, it's not surprising that it is not to be found in any of the standard Linux distributions. Fortunately, it's fairly easy to build from source. First, install the fltk (Fast Light Toolkit, a C++ GUI toolkit) development libraries. I installed version 1.1.5 of fltk and fltk-devel from the Dag rpm repository.
You will also need X11 and ghostscript installed on your system, but everyone has those these days. Then, simply untar the flpsed source and run make to build flpsed. After this completes successfully, copy the flpsed binary to a convenient location such as /usr/local/bin/.
The Xfig image editing program and comes standard on most Linux desktop systems, so I won't cover it in much detail here, other than to point you to the Xfig home page for more information.
Flpsed only operates on postscript files, so the first thing you have to do is convert your PDF to postscript. To do this, use the pdf2ps commmand-line too which comes with ghostscript. This is usually painless since the PDF markup language is a subset of postscript. After running pdf2ps on your file, you will find a postscript file with the same name and .ps extension in the same directory. When you are done editing the file, you can convert it back to PDF with the corresponding ps2pdf utility.
Flpsed is a completely graphical application, so run it from your desktop and use the file menu to open the postscript version of your PDF. After a bit of churning, flpsed will present you with the first page of your document in a window. You can now edit it as you want. Click on any area of the page you wish to add text and type away. You can use the arrow keys at any time when you have a textbox open on screen to reposition the text in small increments. This allows you to line up text accurately.
The flpsed user interface is very primitive. You can only add text to a page: once you add a text fragment, it remains forever (although you can make it empty). There's no way to add other graphical elements such as circles or boxes. You only get one font: courier, for that "I used a typewriter" look, in 3 sizes.
At least on my system, the font displayed in the flpsed editor did not match the font my printer used. This made exact positioning of long strings of text difficult as they tend to run off the page when printed.
You can press <ctrl-n> at any time while editing to move on to the next page. Any additions you've made to the previous page will remain. One big annoyance I had with flpsed is that you can only move to the next page with <ctrl-n> or press <ctrl-p> to skip to the first page of the document. There is no way to move to the previous page or jump to an arbitrary page in the document. This means that if you accidentally skip past the page you want in a long document, you have to start from the very beginning.
Once you are satisfied with the additions you've made to a document, you can save or print it. The save feature writes the file out with your changes embedded as additional postscript. This means that your changes are an integral part of the document and you can pass easily pass it on to anyone as a PDF file (you have to of course turn the postscript back in to pdf with pstopdf first). As I mentioned above, flpsed is most definitely alpha-quality software. It did crash once when I used it. Also, I would occasionally see a glitch where the text I added to a particular page of a document would be printed again on an additional blank page.
Time for Xfig
The Xfig method of editing pdf files is decidedly less elegant than what I've described above. However, it has one big advantage: you can draw anything you want on top of the original document. I was forced to use the Xfig method with some forms recently because the form fields contained default values.
It's a bit of a stretch to say you use Xfig to edit a postscript file. What you actually do is load the original file into Xfig as an image and create an overlay Xfig image on top of that. One major limitation with this approach is you can only operate on a single page of a postscript (or converted pdf) file.
I won't go over the details of using Xfig to add text on top of a postscript page because this howto provides a good description. This method does work to add text and other graphical elements on top of your pdf. However, keep these limitations in mind:
Putting It All Together
Using the combination of flpsed and Xfig, I was able to edit a number of different PDF documents. I was then able to either print the resulting documents with my text included, or email the PDFs to others. Thus, mission accomplished.
However, as you can see from the many caveats above, this is a cumbersome process. I sincerely hope that in particular flpsed is improved and expanded. For example, if it included basic drawing tools, I would not have had to use Xfig.
With a little bit of work, flpsed could be a "killer app" for Linux. Imagine the scenario: a Windows user comes to you with a look of panic on his face: "Help! I've got a PDF I have to edit. What can I do?"
"No problem," you say, as you fire up flpsed and save the day. Score one for Linux.