This is a security setting that you can change in the Settings tab.
Please select the appropiate files access permissions before adding the files to your document for others to see/edit/open.
If you added the files and you want to change the permissions, please open the Folder Tools* sidebar and use one of the tools for changing the access type in bulk.
You can also do this manually by going to Google Drive, selecting the files you want to share, and changing the share settings for the files.
*paid tool.