If you use a MAC you're in luck, as by default your operating system includes a program that allows you to generate files in PDF format. But on Windows things are a bit more complicated, as you need to have a program installed that lets you convert printable documents into portable ones (PDF). Such programs are called PDF Creators/Converters/Writers, and the good part is that there are quite a few available for free.
doPDF stands out from the crowd because it has the tiniest resource usage level. You simply download a 4Mb file and within less than a minute you're ready to create a PDF file. After using it for a while, these are the advantages I see doPDF has over other PDF creators:
It's really free, even for commercial purposes. In comparison, PrimoPDF (another alternative) uses the OpenCandy installation system, which basically suggests other programs to you while installing PrimoPDF. CutePDF installs the "Ask.com toolbar". With doPDF you don't have any problems since it doesn't install (or suggest installing) anything additional than what's necessary.
No GhostScript. Most of the other free PDF creators are using GhostScript, which basically is the core that does the PDF conversion. While this might seem that's not a problem, it means that you have to download another thing and install it in order to be able to create PDF files. doPDF uses its own PDF libraries so not needing GhostScript or .NET installed.
Resource friendly. You'll barely see the dopdf.exe process increase CPU usage in task manager during conversion, as it uses insignificant resources to generate PDF files.
In terms of actually using it, once doPDF is installed you have two choices for creating PDF files. You can open the document you want converted and go to File-Print and select doPDF as a printer, or you can go in your Programs and open doPDF.exe which will start a sort of conversion wizard. So creating a PDF is very easy.
If you need to, you can also change various options in doPDF. For instance, you can change the format of the resulting PDF file (portrait/landscape, custom page size), you can create smaller PDF files (but lower image quality) or you can create high quality PDF files (but larger in size), you can choose to open the PDF once created in the default PDF reader and lastly you can embed fonts in the resulting PDF file (which ensure the PDF will look the same on other computers, but the size of the PDF will increase).
Basically the decision you have to take when creating a PDF file is if you want to leave the default settings as they are, meaning PDF files will be smaller but images might have a quality loss, or if you want to create a PDF with an increased image quality. Either choice, doPDF gives you the option right on the window that lets you save the PDF file, so you don't have to hunt down for these features elsewhere.
doPDF is one of those programs that will always come in your mind when you'll think of PDF files. Not because the name is that catchy, but because after you use it once you'll be happy to have found it.