Hi reziman,
I use either Adobe Illustrator or Adobe Illustrator, depending on what specific part I'm working on. Photoshop is very good for making transparent gifs so that I can take something like a picture of a shotgun and remove the background from it. Illustrator is my workhorse. I clean up illustrations, do the text, etc. and then save as a PDF. Pretty much any paint or illustration program will let you do some level of editing provided that it can read the PDF file. How you save it is less relevant unless you plan to share it.
If you have other questions, post away.