Short answer: Yes, You can use both. They serve different purpose and can be used in the same application.
According to NextJS docs:
Next.js uses the App component to initialize pages.
To override, create the
./pages/_app.jsfile and override the App class
and
Pages in Next.js skip the definition of the surrounding document’s markup. For example, you never include
<html>,<body>, etc. To override that default behavior, you must create a file at./pages/_document.js, where you can extend the Document class.
Note: _document.js is only rendered on the server side and not on the client side. so event handlers like onClick is not going to work.