My site got hacked last month and a lot of PDFs were added in Google ending in /index.php?voxter.pdf&gpvoq=1&static=1866`.

I have cleaned the site but now these pages are redirecting to home page instead of returning error 404. As a result, these pages are still there in Google Search results. Whenever I use /index.php?voxter.pdf&gpvoq=1&static=1866 this at any WordPress site, it shows homepage only.

But my question is why Google is showing these non existent pages in search results?

One argument can be Gogle previously indexed these pages so it wont remove it from results till they return error 404. How can I make these pages to return error 404?