While this is not a free PDF, it costs around $15–$20. Considering the hours of studying, this is a wise investment.
You will find many links on Reddit, Scribd, or random study blogs promising a "free minna no nihongo n4 renshuu b answers pdf."
Ask yourself:
The PDF format allows for use on multiple devices (tablets, phones, PCs). Why the PDF helps: This allows for "study on the go." You can review the drills on a commute or during a break without carrying the heavy physical workbook and separate answer key booklet. It also allows for quick keyword searching (Ctrl+F) if you are looking for a specific grammar point or vocabulary word within the answers.