The document structure of PDF and Microsoft Word are totally different. Everything in PDF is fixed, when you create PDF, all the elements in Word doc will be located in a certain place in PDF document, that's why PDF files can keep the original look in different operation system, screen size, etc. Microsoft Word can save as PDF.
While you convert PDF to Word, Word doc is reflowable so it's hard to locate elements in exact location, so the original format may lost. Even the Acrobat can't convert PDF to Word 100% accurately. Output quality depends on the developer's algorithm. Solid Document works great on Windows, while NitroPDF provide online converter pdftoword.com, Cycle PDF Converter for Mac is great, and if you want to convert scanned PDF to editable Word doc, ABBYY's OCR can help.