PDF stands for "Portable Document Format". It is a file format create by Adobe that preserves the layout, fonts, images and graphics of the document. By creating a PDF file from your book document you are guaranteeing that the person who views that file is seeing it exactly as you intended it to be seen. For even more information about PDF visit the Adobe PDF information page.
In order to create a PDF file you first need to create your document using the word processing software or application of your choice such as MS Word. Once you have your book document looking exactly as you want it you would then convert it into a PDF file using a PDF writer program. The most popular PDF writer is Adobe Acrobat. Their program is considered one of the best but it is expensive. There are some other PDF writers out there that are more affordable if you are on a budget.
Some other options are:
OfficeReady Professional 3.0
Affordable software for MS Office that enables you to create a PDF file from
Microsoft Word, Excel, PowerPoint & Publisher files.
pdfFactory
Affordable PDF writing software that allows you to create a PDF from any
application.
OpenOffice
A free open source word processing program with a built in PDF
writer.
PDF995
A free PDF writer that works from any application.
Doing a search on Google for "PDF creator" or "PDF writer" will bring up even more options.
Be sure that the writer you choose has the ability to embed fonts within the PDF file.
Some publishers and book printers are starting to only accept PDFs created with Adobe Acrobat due to some problems with PDFs created with other cheaper applications. Check with your printer or publisher to see if they have any restrictions on what PDF software you may use.
Another option is to use Adobe's "Create Adobe PDF's Online" system which allows you to upload a document to their online PDF converter. They have a free trial which you can use to convert up to 5 documents or a monthly subscription which you can convert unlimited amount of documents. I have uploaded a test book document that was about 300 pages and it did not have a problem with a document of that size. However it did have a problem with my custom book size (6x9) in that it placed it on a default 81/2 x 11 size paper. They have a solution for this at: http://www.adobe.com/support/techdocs/328610.html.