When I send an e-mail with an attachment, the receiver gets a link that cannot be opened, not the attachment.

Just figured it out. If you use Preview as the default reader and attach the pdf file the other person cannot open it since they probably do not have Preview. When I send the pdf file using Acrobat Reader as the default the other person can then open the file.

The link is caused by sending a large file with MailDrop.

I don't know why they cannot download the file unless it has been more than a month since it was sent. The links do expire. The link should just directly download the attachment. Perhaps they have some security software that prevents the link from working correctly.

Is it possible you attached an alias to the email rather than the original file?

I also just started to have this issue with Monterey. The receiving person states cannot open the attachment. I found if I send it from my Web email site then it is OK.