I'm having the same issue.
I go to Sales then Customers, I search for the company and the choose Edit. I have attached the document (pdf) in numerous ways and it looks good. I save and exit. When I go back into the company to ensure the attachment is there and attempt to open that document, I get the exact same error as Maybelle.
The pdf uploaded is a signed contract.
This XML file does not appear to have any style information associated with it. The document tree is shown below.
<code>INVALID_AUTHORIZATION</code>
<type>CLIENT</type>
<message>Authorization header is missing</message>
</DocumentErrorResponse>
