Add or modify Text to the PDF in Adobe Acrobat

To add or edit text to a PDF file using the Adobe Acrobat, select "Tools->Advanced Editing->Touchup Text Control", Now select some text to modify, a rectangle will appear, and one can begin typing text, or modify the font by right -click the selected text and select "Properties".

To add a signature or draw something on PDF, select "Tools->Drawing Markups->Pencil Toos"

Free tool for file conversion between PDF, Tex, Word and so on

pandoc (Download from http://johnmacfarlane.net/pandoc/index.html)