No idea, how you would get them out. But then it depends on the content type and the meta data you want to preserve.

You probably need to implement some kind of importer. e.g. for documents, you would need to put the metadata + files somewhere and then implement a service that reads metadata and files and uses DLFileEntryLocalService (and maybe DLFolderLocalService) to create files and folders in the system. If you need e.g. tags/categories, you can add them too in that case.

https://docs.liferay.com/portal/7.0/javadocs/portal-kernel/com/liferay/document/library/kernel/service/DLFileEntryLocalService.html

If you need no metadata, you might even be able to import them using webdav. Then it would be a simple copy & paste operation.

But it really depends on the details. It is really impossible to give you an answer in a forum posting to such big question.