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DiffPDF Portable was developed as a simple, accessible and easy-to-use piece of software that can help users compare two PDF files.
DiffPDF Portable comes with three comparison modes: Words, Characters, 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.

Via: DiffPDF Portable 2.1.1