You have two different document formats here: Word is a word processor, PDF is text layout.
They may look a lot alike, but there's a very big difference.
In a PDF, the page layout is fixed (mostly - there's a format called "reflowable PDF"). The sizes and positions of everything are quite firmly nailed down onto each page of the document.
In a Word document, the page layout is more fluid, as anyone who's taken a Word document and moved it to a different computer - or even opened it with a different word-processor such as LibreOffice - can attest. Paragraphs slide around from page to page. Fonts don't always match (this used to be a major problem before FreeType).
So at a minimum, expect to lose some things when converting.
A PDF is a set of metadata combined with a series of PostScript commands. A Word document can be represented using Rich Text Format in a plain text file, or in traditional .doc format or in the XML-based .docx format. The command set is the same, regardless, so only the notation varies.
There are some websites that claim to be able to convert PDF's or PostScript to Word format. Personally, I prefer something I can run in-house, for security reasons. But I haven't been able to actually find anything like that.
Linux has a very rich set of document-processing tools, so it's possible I could pipe a few of them together to do what you want, but no immediate solution comes to mind.