Added pdf-related notes to the knowledge base

knowledge base/compare two pdf files.md

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+# Compare two PDF files
+
+Install and use `diffpdf` (preferred) or `diff-pdf`:
+
+```shell
+sudo zypper install diff-pdf
+```
+
+As an alternative:
+
+```shell
+# create a pdf with the diff as red pixels
+compare -verbose -debug coder $PDF_1 $PDF_2 -compose src /tmp/$OUT_FILE.tmp
+
+# merge the diff-pdf with background PDF_1
+pdftk /tmp/$OUT_FILE.tmp background $PDF_1 output $OUT_FILE
+```
+
+## Further readings
+
+- [Compare PDF Files With DiffPDF]
+- [PDF compare on linux command line]
+
+[compare pdf files with diffpdf]: https://www.linuxandubuntu.com/home/compare-pdf-files-with-diffpdf-in-ubuntu-linux-debian-fedora-other-derivatives
+[pdf compare on linux command line]: https://stackoverflow.com/questions/6469157/pdf-compare-on-linux-command-line#7228061