Cannot attach PDF documents to MacMail

Can anyone offer any advice on how to fix this:

On only 1 profile (of 4 on our family's MacMini), we cannot attach PDF documents to MacMail or any web-based e-mail interface (i.e. gmail or aol). We're using the latest version of Mac OS and Adobe Acrobat Reader DC with all updates current. Other attachments work without issue (jpg, docx, pages, xlsx, etc). Any advice is appreciated! Thanks,