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 fileAn universal picture and document converter
Online converter for PDF documents
A web-based OCR conversion and text recovery service
One of the many PDF converters