I was recently sent a pdf and because the text was over a block of colour I was unable to copy and paste from it as it was unable to see the text for me to select it. Just before I was about to type it all up I thought “OCR”+”web apps”, there has to be something, and sure enough there was 
www.onlineocr.net

I am sure there are loads more but this one was free and did what I needed.