I cann't find the "define" option in adobe reader when I read a PDF file in my iPad 3. I have already latest reader version and iOS 5.1.1 installed.
Can I ask where you open the PDF file? I open the file in adobe reader and when I tap and briefly hold it foe few seconds, I can only see "note, text, freehand , signature". I can't see " define" or " copy"....
I believe that I figured it out.
holding finger over a word, will bring the copy/define menu
holding finger where there is NO text, empty space or image, will bring up the annotation menu.
but note that if you have a scanned document, where the words are actually an image, you will get the annotation menu, since there really is no text, just a bunch of pixels that look like text...
you can turn scanned text into "real" text by using OCR functionality of the desktop version of Acrobat Pro (not Reader), it would then create a layer of invisible text under the image of the text.