merge pdf's -> https://stackoverflow.com/questions/4794435/merge-pdf-files-with-php

https://tcpdf.org

https://www.ghacks.net/2007/10/02/merge-pdf-files/

https://www.pdflabs.com/tools/pdftk-server/

composert -> http://culttt.com/2013/01/07/what-is-php-composer/

fdpi -> https://manuals.setasign.com/fpdi-manual/v2/installation/#index-2-1