If you want to send your PDF files (Invoices, Purchase orders, ...) via erplain, simply click on the Actions button directly from the transaction and click on "Send by email". Erplain will use its own email servers to send the email with the PDF as an attachment.
You can create email templates to avoid retyping the same information each time. You can find more information on this page: Email templates.
The email will be sent from a standard erplain email address: "firstname.lastname@example.org" but your username will appear as the "From" of the email. If your customer replies to the email, the "reply-to" address will be your user email address.
If you prefer to use your email system to send emails, you can set it up in your user profile. Go the User menu and click on Edit my profile:
Scroll down to the SMTP Configuration section and type your SMTP settings. You can usually find your SMTP settings from your email system by searching for "Outlook STMP" or "Office 365 SMTP" for example:
If you are using Gmail, please refer to this article: Gmail SMTP setup.
Once you have set it up, your emails will be sent from your email account.