18.04 – How to merge multiple PDF files into one in Ubunutu?

Yes, pdftk, in the pdftk package.

Of pdftk man page:

            If PDF is an electronic document, pdftk is a device for staple removal, punching, binding, secret decoder ring and X-ray glasses. Pdftk is a simple tool for managing PDF documents on a daily basis. Use it for:

* Merge PDF documents or assemble PDF scans
* Split PDF pages into a new document
* Rotating documents or PDF pages
* Decrypt the entry as needed (password required)
* Encrypt the output as you like
* Fill in PDF forms with X / FDF data and / or flattened forms
* Generate FDF data templates from PDF forms
* Apply a background watermark or a prominent stamp
* PDF metric reports, bookmarks, and metadata
* Add / Update PDF bookmarks or metadata
* Attach files to PDF pages or PDF document
* Unpack the PDF attachments
* Break down a PDF document into single pages
* Uncompress and recompress page feeds
* Repair corrupted PDF (if possible)