This is most likely to be because the spec sheets are a protected from "Document Assembly"
You can check this by opening the PDF in Adobe
Then from the toolbar Select EDIT > PROTECTION > SECURITY PROPERTIES
Click the SECURITY tab
Under - Document Restrictions Summary
Check the status of "Document Assembly" if it says "Not Allowed" you will NOT be able to add this PDF to your quote.
Some manufacturers do this to protect their intellectual property.