Add notes on pdftk for PDF manipulation

- Included basic commands for common PDF tasks using pdftk
- Added instructions for:
  - Merging PDFs
  - Appending one PDF to another
  - Cutting specific pages from a PDF
  - Splitting PDFs into individual pages
  - Rotating PDF pages
  - Encrypting PDFs with a password

This commit provides essential pdftk commands for managing and manipulating PDF files.

notes/pdftk.md

@@ -0,0 +1,57 @@
+# pdftk
+
+## Table of Contents
+
+- [pdftk](#pdftk)
+  - [Table of Contents](#table-of-contents)
+  - [Basic Commands](#basic-commands)
+
+## Basic Commands
+
+**Merge PDFs**
+
+To merge two PDFs, use:
+
+```
+pdftk file1.pdf file2.pdf cat output merged.pdf
+```
+
+**Append PDF**
+
+To append one PDF to another, use:
+
+```
+pdftk A=first.pdf B=second.pdf cat A B output combined.pdf
+```
+
+**Cut pages from PDF**
+
+To only keep the first page:
+
+```
+pdftk file1.pdf cat 1-1 output output.pdf
+```
+
+**Split PDF**
+
+To split a PDF into single pages, use:
+
+```
+pdftk input.pdf burst output page_%02d.pdf
+```
+
+**Rotate PDF Pages**
+
+To rotate pages in a PDF, use:
+
+```
+pdftk input.pdf cat 1-endsouth output rotated.pdf
+```
+
+**Encrypt PDF**
+
+To encrypt a PDF with a password, use:
+
+```
+pdftk input.pdf output encrypted.pdf user_pw yourpassword
+```