PDF files are difficult to compare using open source / free software. But it is somehow possible. Here is how i do it...
First i use "diffpdf" version 2.1.1 to get visual insight (which is still free, unlike the never versions - however, not opensource). For the rest i do not like this application because it cannot re-align, misses some differences, and is not open / free.
Next, i use "WinMerge" with the "xdocdiffPlugin" to extract text and compare.
An issue that you often see with PDF is that the diff is mis-aligned due to page breaks (page headers and footers in the text version).\\ To resolve this i create txt version from the WinMerge / xdocdiffPlugin output and use "KDiff3" to do some "manual diff alignment". Alternatively, you can remove some new page header/footer text but that is bothersome, i do not like information loss.
A final step is that i use "KDiff3" to print the entire visual diff, in color, to PDF in landscape format.
Oh, i wish i could do everything with one tool: