Hi Judy,
Many programs (Gimp, Inkscape, MS Word) have the option to "print" to PDF. To use this option you must first have a PDF "printer" installed. I use a freeware program called CutePDF
http://www.cutepdf.com/ .
Once installed, when you click on Print from a program like Gimp, you can select the printer you want to use from a drop-down list. CutePDF will be one of them. It works well for me.
Hope this helps,
Jim