Not only do these things lack any significant security benefit. One thing that's underappreciated is the strong security disadvantage and imbalance of power it creates against the recipient. They have no means of ensuring the document behind the "secure link" is the same as it was at the moment the email arrived and that it has not been alterred by the sender or some third party with access to the "secure document" platform between the time the email was sent and when they read (or later re-read) it. If the document were included as an attachment, the recipeint would possess a permanent copy from the moment of receipt, and depending on how DKIM was used, possibly even a cryptographic signature establishing authenticity/non-repudiation of the document.
As noted by bta in the comments, another way these things harm the recipient's security is by bypassing any scanning for malware, etc. that would be done for direct attachments but not for off-site links. And as ThoriumBR seems to have suggested, these kinds of "secure links" may train recipients to get phished - by normalizing following links to documents and possibly entering private information or performing authentication processes with the site.
