DiffPDF is developed by Mark Summerfield, I just compiled it to windows 32 version.
DiffPDF depends on poppler, poppler-qt, Qt4 library.

DiffPDF can compare two PDF files. It offers two comparison modes: Text and Appearance.
By default the comparison is of the text on each pair of pages, but comparing the appearance of pages is also supported (for example, if a diagram is changed or if a paragraph is reformatted). It is also possible to compare particular pages or page ranges. For example, if there are two versions of a PDF file, one with pages 1-12 and the other with pages 1-13 because of an extra page having been added as page 4, they can be compared by specifying two page ranges, 1-12 for the first and 1-3, 5-13 for the second. This will make DiffPDF compare pages in the pairs (1, 1), (2, 2), (3, 3), (4, 5), (5, 6), and so on, to (12, 13).

It is open source(of course free software), want to try it on your windows XP/Windows 7, please download here.

• View PDF, PPt and Tiff online with Google Docs, even you do not have a Google Account or you do not want to login it. and you can embed the online documents into your own web pages.
• Google Docs add OCR support to PDF and Images, so you can OCR your scanned PDF, Fax, Images without any fee.
• Built-in PDF Reader for Google Chrome, view PDF just like view html. Chromium’s blog announced that the latest Google Chrome dev build for Windows and Mac includes a plug-in for viewing PDF files. The plug-in can be enabled by going to chrome://plugins/ and clicking on “Enable” for the “Chrome PDF Viewer” plug-in.
  When you click on a link to a PDF file, Chrome no longer opens the file using the Adobe Reader plug-in. Instead, Google Chrome uses a basic PDF viewer that lacks many useful features like pagination and bookmarks.

From now on, you can freely OCR your scan PDF documents, Images and Fax online with Google Docs.
When you upload files to Google Docs, you’ll notice a new option that tells Google to convert the text from PDF and image files to Google Docs documents. The feature has been released last year as an experiment, so Google had enough time to improve the accuracy of the results.
I’ve done some test and the result wasn’t great. About 10% of the text has been incorrectly converted and the formatting hasn’t been preserved.

“This document contains text automatically extracted from a PDF or image file. Formatting may have been lost and not all text may have been recognized,” explained Google in a note included in the document.
This feature only works for the following languages: English, French, Italian, German and Spanish. “For the technically curious: we’re using Optical Character Recognition (OCR) that our friends from Google Books helped us set up. OCR works best with high-resolution images, and not all formatting may be preserved.”, Google Docs Blog says.
btw, another good news, with Google Docs, you can freely view online PDF, PowerPoint and Tiff, without need to login your Google Account or download them, you can also embed them in your own web page. for details, please visit here.