Added pdftk notes to the knowledge base

knowledge base/combine multiple pdf files.md

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+# Combine multiple PDF files
+
+## Further readings
+
+- [pdftk]
+
+[pdftk]: pdftk.md#combine-multiple-files

knowledge base/pdftk.md

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+# PDFtk
+
+## TL;DR
+
+```shell
+# combine multiple files
+pdftk file1.pdf file2.pdf file3.pdf cat output newfile.pdf
+
+# rotate a file
+pdftk file.pdf cat 1-endleft output newfile.pdf
+```
+
+## Combine multiple files
+
+```sh
+pdftk file1.pdf file2.pdf file3.pdf cat output newfile.pdf
+```
+
+where:
+
+- `file{1..3}.pdf` are the input file
+- `cat` is the operation on the files
+- `output` is the operation after the read
+- `newfile.pdf` is the new file with the result
+
+## Rotate a file
+
+```sh
+pdftk file.pdf cat 1-endleft output newfile.pdf
+```
+
+where:
+
+- `file.pdf` is the input file
+- `cat` is the operation on the file
+- `1-end` is the range of pages on which execute the rotation
+- `left` is the direction of the rotation
+- `output` is the operation after the read
+- `newfile.pdf` is the new file with the result
+
+## Further readings
+
+- [Combine multiple PDF files with PDFTK]
+- [Lossless rotation of PDF files with ImageMagick]
+
+[combine multiple pdf files with pdftk]: https://www.maketecheasier.com/combine-multiple-pdf-files-with-pdftk/
+[lossless rotation of pdf files with imagemagick]: https://stackoverflow.com/questions/38281526/lossless-rotation-of-pdf-files-with-imagemagick/51859078#51859078