Adding text to a .pdf file

I have an official document that I have scanned in to a .pdf file, and I want to add some text that says 'I certify that this is a true copy of this document' (I do not want to change the document itself, and this will save me having to write this text on 20 pages).

Is there a simple (and cheap) way of doing this, short of buying Acrobat Pro?

What have your google searches yielded? I found this:

How to edit PDF forms without Adobe - YouTube

I have no idea if it works.

Hello Jcaswell,

You might see if the free program Nitro PDF Reader may allow you to be able to do this.

Hope this helps,
Shawn

I use PDF-XChange Viewer. The free download allows text input, though it's a bit limited, no choice of font or font size. On the toolbar, Tools > Comment and Markup Tools > Typewriter Tool gets you there.