Update cheatsheets

pdfunite

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# pdfunite
+
+> PDF merging utility.
+> More information: <https://github.com/mtgrosser/pdfunite>.
+
+- Merge 2 PDFs into a single PDF:
+
+`pdfunite {{path/to/fileA.pdf}} {{path/to/fileB.pdf}} {{path/to/merged_output.pdf}}`
+
+- Merge a directory of PDFs into a single PDF:
+
+`pdfunite {{path/to/directory/*.pdf}} {{path/to/merged_output.pdf}}`