You just need an app that supports PDF files, which the built-in iBooks app (unless you've deleted it) will do e.g. Use iBooks with PDF documents on your iPhone, iPad, or iPod touch - Apple Support
If you don't want to use iBooks download a different PDF app from the App Store, if the app supports it you should be able to copy PDFs to it similar to above or File Sharing via your computer's iTunes : About File Sharing on iPhone, iPad, and iPod touch - Apple Support