A family of Microsoft word processing software products for creating web, email, and print documents.
You haven't actually changed the PDF files to Word files. The only thing that happened is that you made Word the default program for Windows to start when you open a PDF file -- it's a setting in the registry.
There are a couple of ways to fix this.
The easy way is to right-click any PDF file in File Explorer, click the Open With item in the menu, and select your PDF reader program such as Adobe Reader) and click the "Always use this app" option. That will make the PDF reader the default program for PDF files. You'll still be able to open a single PDF in Word if you want to -- though it's probably better to start Word and then use the Open dialog (Ctrl+O) to select the PDF.
The longer way is to press Windows key + i to open the Control Panel and click Apps; click "Default apps" in the left column; scroll to the bottom of the page and click "Choose default apps by file type"; scroll the list down to .pdf; click the Word icon on the right; and choose your PDF reader program from the list that pops up.