PDF is a widely used format that’s ideal for distributing materials that should not be editable and is also virtually universally accessible (there are free programs to read PDF’s on all major operating systems). For more information on the format, please see the Portable Document Format wikipedia entry.
There are numerous programs to create PDF’s. The standard is Adobe Acrobat Professional, which can both edit existing PDF’s, and also convert existing documents from other formats to PDF, but it is not free.
Depending on your situation, you may in fact be able to get by with the various free options out there. For instance, if all you want to do is be able to convert existing documents into the PDF format, you can use a free program such as PDFCreator. Another free alternative is CutePDF.
While these can split PDF’s into multiple files, join others into a single file, and rearrange the pages, unfortunately, there is no free option that can edit the actual content of a PDF. If that is the capability you require, your only options are commercial software such as Adobe Acrobat Professional (which can be purchased for a discount from HiEd) or Foxit PDF Editor which can be downloaded and evaluated before purchase (the evaluation version places a “watermark” on any saved PDF’s so that it cannot be used for production).