You cannot convert a graphic (bmp) image to a simple txt file. To convert the text in a pdf file to a txt file open the pdf in Acrobat Reader and select Tools > Basic > Select. Next select the text you want to copy. Finally, select File > Save As Text.
I use Adobe Reader 7.0. If Tools > Basic > Select does not allow you to select the text then it must really be a graphics image. In that case you'll probably need OCR (Optical Character Recognition) software. Such software can be rather expensive and I don't know which package to suggest.