Added notes about diffpdf to the KB

knowledge base/compare two pdf files.md

@@ -3,9 +3,14 @@
 Install and use `diffpdf` (preferred) or `diff-pdf`:
 
 ```sh
+sudo pacman -S diffpdf
 sudo zypper install diff-pdf
 ```
 
+```sh
+diffpdf file1 file2
+```
+
 As an alternative:
 
 ```sh

knowledge base/diffpdf.md

@@ -0,0 +1,11 @@
+# DiffPDF
+
+## TL;DR
+
+```sh
+# Install it.
+sudo pacman -S diffpdf
+
+# Show differences visually.
+diffpdf file1 file2
+```