How to write text on PDF files for free?

If you are looking to write text on PDF files or fill up forms, try:
http://www.pdfescape.com

It's a free online PDF editor, and will let you type on the file where-ever needed.

Here are its major features:

Free Online PDF Reader
Open PDF documents natively in your web browser
Rotate & zoom PDF pages to preferred viewing style
Select text and copy PDF content to your clipboard
Save, download, email, & print PDF documents
PDF thumbnail, bookmark, & link support

Free Online PDF Editor
Add text, shapes, whiteout & more to PDF files
Move, delete, & insert PDF pages
Create links to other PDF pages or web content
Change PDF information tags
Encrypt PDF contents using a password
Add & edit PDF annotations (sticky notes)

Free PDF Form Filler
Fill out PDF forms using existing form fields or use text tool
PDF text, checkbox, radio, list, and drop down fields supported
Essential PDF field calculation and formatting supported
Basic PDF field styling properties supported
Quickly tab from field to field

Free PDF Form Designer
Add new PDF form fields to any PDF file
Style PDF form fields (font, size, color, etc)
Modify existing PDF form fields

If you want more advanced features, the same company developed another application:
http://www.pdftypewriter.com

Price for this one is less than $30.