The latest Microsoft Word now can directly open .pdf files and thus save them as .docx documents. Alternatively, you can also use some of the dedicated document conversion programs that should be able to do pdf to docx conversion just fine.
Adobe Acrobat Reader DC (from Creative Cloud subscribtion service) can save documents either as text, or in Microsoft Word .docx and Excel .xlsx formats, so with that it should be quite simple to convert PDF documents to DOCX format.
Except for that, plenty of programs nowadays support handling of PDF documents and exporting them to all kind of other formats, which means you can use any of them for pdf to docx conversion.Open PDF file Open DOCX file
A dedicated PDF conversion solution
An universal picture and document converter
Corel WordPerfect is word processor for Microsoft Windows
Online converter for PDF documents
A web-based OCR conversion and text recovery service
One of the most popular word processor program from Microsoft Office suite
Professional editor of PDF documents
An Office document editing app running on S60 based smarthones
One of the many PDF converters
A Korean word processor
A tool for converting PDF files to other file formats