There are no file size restrictions for any of the plugins. However, some mail servers do block emails with attachments over a certain size. It’s possible that’s the issue. You could find or make a very small PDF file and test it — that will tell you for sure if it’s a file size issue or if perhaps it’s something else.
Did you use the same test email address when you used a doc file as when you used a pdf file? Note that many email providers will block email if the email address in the From: header is different than the domain name / account of the system sending the email. That is a common problem when your website is hosted on a 3rd party provider. People usually fix that by using the Postman SMTP program or by installing Custom Sender For Email Before Download — either of those will let you set the From: value of the email to be a valid email address for your domain (which will fix the problem of some email providers, like Yahoo Mail, blocking the incoming email). That could also be the problem, particularly if you used different test email addresses for the doc and the pdf when you ran your tests.
