When a client receives a document, the default setting for the email button text is
"Read Your Document"
This can be edited under your brand settings - just click Settings > Setup > Branding in the sidebar on the left.
Next, choose the brand you want to edit.
Click the 'Email' on the little horizontal menu of options and you’ll find the option to edit the current text of your email button.
You might want to edit this text if you use Better Proposals to send other types of documents (for example Reports, Statements of Work, or Digital Sign-off).
So changing the button text to "Digital Sign-Off Enclosed" here...
...will show up on the email when your client receives the document.
For each different type of document, we would recommend duplicating your brand and having one set to "Read your document" and one set to "Read your report" for example.
You can add a new brand by clicking the 'Create New Brand' button (more information here).