The PDF opens without a password in Adobe, but I cannot open in PDFNet.
According to Adobe "all the contents of the document are encrypted" and the Security Method is "Adobe Experience Manager Document Security".
It looks like the file is encrypted with Adobe Experience Manager Document Security, which is proprietary security.
Therefore, the PDF is technically a broken/malformed PDF file, that can only be handled by Adobe products.
When you call InitSecurityHandler you will get the following exception.
Requested security handler was not found.
Which means the PDF is using a proprietary encryption, that is not defined in the PDF standard.
A normally encrypted file would just return false.
In that catch block you give a message that the file appears to have proprietary security.