That option exports form fields and its values. This is for example an input field or a dropdown field, if you open the PDF with any program such as Adobe, Foxit or Google Chrome you can change both fields (input and dropdown) and save the changes.. It needs using structure document tags (https://www.phpdocx.com/api-documentation/word-content/add-structured-document-tag-Word-document-with-PHP).
You can test it manually using LibreOffice. You can open the DOCX with LibreOffice and then choose File->Export As->Export As PDF , in the next window you need to enable Create PDF form (FDF format). phpdocx does the same process but with the transformDocument method and a single line of PHP.