If Open does not work, but Get Unmarked PDF does, verify the original document to make sure there are no security measures preventing it from being modified.
When only the "Open" operation does not work, but the "Get Unmarked PDF" does, this usually happens when the original file is already a PDF that is password protected.
Password protected files are not supported because, when the Publishing Type of a document is set to "Publish in non-editable format (PDF)", the system will try to convert the original file and generate a new file by appending the SOLABS QM10 cover pages and/or adding headers and watermarks (depending on the conversion settings associated with the document type of the document).
Since in this situation, the original file is already a PDF, it will not be converted by the conversion engine, and will be directly used as the basis to generate the output file. Because it is password protected, it will not be possible for the system to append the cover pages or set the header/watermark, which prevents the "Open" operation from working as expected.