The answer your first question, no, Forms cannot use a variable for the From address. Depending on how your email system is configured, there's a chance it would get blocked even if it could. For example, if you attempt to send "from" the email of the user who submitted the form, most email servers would view that as spoofing unless you made special rules to allow it.

To answer your second question, Forms only has access to variables associated with the business process, so you cannot set a static attachment. However, you might be able to do both of these things with a Workflow instead.

In Workflow, you can add an email service task, and under the advanced settings you can set a Token as the email address (be sure to select the correct SMTP server). It won't show the token menu, but if you enter a valid token for the sender address it will work.

For the static PDF, you could put it in a repository then use Find Entry to retrieve it based on the Entry ID, use Download Electronic File to get the PDF, then set the PDF as the attachment in the email activity.

In the workflow properties, check the box that says "Started by Laserfiche Forms" and add a Retrieve Business Process Variables to grab the form values. Then, you just call that workflow from inside of your Forms business process.

However, I must reiterate that depending on what email address your trying to send as, there's a chance it could be rejected by the email server (or servers if you're trying to send out) because Forms may not technically have rights to send on behalf of that user.