Documents can have attached files. Key names of attachments can be different according to the solutions.
Document's property of single attachment looks like this:
Download this file you can by downloadURL
Document's property of multi attachment looks like this:
Use all links at "downloadURL" property to download files.
One document can contain multiple attachment attributes. Consultant of Vecticum can give you information, which fields are for attachments, or you can recognize those by structure from example above.
IMPORTANT! Files names often can match each other. If you downloading files into the same folder, make sure to rename it or split it to different folders to avoid overriding them.