Document = is an article of a newspaper which is for PUBLIC view in JPG
Attachment= is the page or all pages newspaper which is for PRIVATE or only for research role view (JPG or PDF)
This is not possible right now, although we do have an issue open for that:
The reason is that the document and attachments visibility are directly bound to the item visibility. It the item is public, both will be public. Metadata, however, can have different layers of access, so you can have a public item with private metadata. We plan in the future to allow having attachments defined as a metadata type.
Meanwhile, you have two alternatives:
To use the URL metadata type (it is a plugin, but soon we’ll be releasing a version where it is part of the core) to place a link to your PDF file in a private metadata;
Store the private files in a second collection and use a Relationship metadata to bind it to the item of you current collection.
I also like to have a backup of everthing online or offline which is the base of everything in case I have to set up the collections again in Tainacan or transfer to another system in the future.
In this case propably I need to store the metadates in spreadsheet (excel or Google sheet) and store the original files in folder.