pdfattach: add page (#14110)

* pdfattach: add page

---------

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

pages/linux/pdfattach.md

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+# pdfattach
+
+> Add a new attachment (embedded file) to an existing PDF file.
+> See also: `pdfdetach`, `pdfimages`, `pdfinfo`.
+> More information: <https://manned.org/pdfattach>.
+
+- Add a new attachment to an existing PDF file:
+
+`pdfattach {{path/to/input.pdf}} {{path/to/file_to_attach}} {{path/to/output.pdf}}`
+
+- Replace attachment with same name if it exists:
+
+`pdfattach -replace {{path/to/input.pdf}} {{path/to/file_to_attach}} {{path/to/output.pdf}}`
+
+- Display help:
+
+`pdfattach -h`
+
+- Display version:
+
+`pdfattach -v`