A family of Microsoft word processing software products for creating web, email, and print documents.
I would recommend at least the free Acrobat Reader from Adobe.
Shalom_409 tells you about opening pdf files in Word; in my opinion you do not want to do this because it converts the file to Word format, generally imperfectly.
The reader will simply open the file for reading, not editing.
Word itself lets you save a Word document to pdf format in two ways.
Save As and choose pdf as the file type, or
Print to Microsoft Print to PDF printer driver.
I have the full Adobe Acrobat which lets me edit pdf files, as pdf files. There are free and lower-cost alternatives available as well if you want to be editing pdf files.