diff-pdf: add page (#7711)

pages/common/diff-pdf.md

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+# diff-pdf
+
+> Tool for comparing two PDFs.
+> More information: <https://github.com/vslavik/diff-pdf>.
+
+- Compare PDFs, indicating changes using return codes (`0` = no difference, `1` = PDFs differ):
+
+`diff-pdf {{path/to/a.pdf}} {{path/to/b.pdf}}`
+
+- Compare PDFs, outputting a PDF with visually highlighted differences:
+
+`diff-pdf --output-diff={{path/to/diff.pdf}} {{path/to/a.pdf}} {{path/to/b.pdf}}`
+
+- Compare PDFs, viewing differences in a simple GUI:
+
+`diff-pdf --view {{path/to/a.pdf}} {{path/to/b.pdf}}`