Usually in PDF, what you can see in the file is the entire look of the page resembling that of a scanned text document. You cannot copy or manipulate the text just like in a word document. In cases like these, conversion of the said file is necessary. Other programs allow you to do that. In addition, another separate program can actually let you compare the PDF files from the original text or the other way around.
Imagine comparing a six hundred-page PDF document to its original form, would you think it would be accurate by just looking at these two files side by side and checking them per page? That would be a bunch of rapid eye movements and perhaps some points of missing out on some details. Most school projects like thesis, dissertations and term papers are now being accepted in a PDF file format. These are not only ten pages or so. It would take your precious time in checking for the details from the original text.
For a faster, easier and accurate way of file comparison, there are available versions of comparing programs for different type of files. A number of comparison possibilities are offered by the program. Comparing PDF file with another PDF file...