For a low/no code solution, you could play around with something like this:

Step 1: Create a Picklist on your custom object (lets call this Template__c on object Invoice__c) which will be used to dictate the Email template, e.g. Picklist values

• Email Template 1
• Email Template 2
• Email Template 3

Step 2: Create your Email template as a Visualforce template, only rendering the relevant content for your attachment, for example:

<messaging:emailTemplate subject="FinancialForce Invoice Generation - {!relatedTo.Name}" recipientType="Contact" relatedToType="Invoice__c">
    <messaging:attachment renderAs="pdf" filename="{!relatedTo.Name} rendered="{!relatedTo.Template__c == 'Email Template 1'}">  
        <c:MyVFForTemplate1/>    
    </messaging:attachment>
    <messaging:attachment renderAs="pdf" filename="{!relatedTo.Name} rendered="{!relatedTo.Template__c == 'Email Template 2'}">    
        <c:MyVFForTemplate2/>    
    </messaging:attachment>
    <messaging:attachment renderAs="pdf" filename="{!relatedTo.Name} rendered="{!relatedTo.Template__c == 'Email Template 3'}">    
        <c:MyVFForTemplate3/>    
    </messaging:attachment>
    <messaging:htmlEmailBody >
        Dear Customer,
        
        Here is your Invoice.  Please pay now, or else!
    </messaging:htmlEmailBody>
</messaging:emailTemplate>

(note, i have illustrated the example using a VF component but you could just as well copy/paste the VF page content into the Email template.

Step 3: Create a workflow rule with the following attributes:

• Object = Invoice (Invoice__c)
• Evaluate the rule when a record is: created, and any time it’s edited to subsequently meet criteria
• Run this rule if the following criteria are met Template__c = 'Email Template 1' or 'Email Template 2' or 'Email Template 3'
• 'Save & Next'

Step 4: Add a new Workflow Action: New Email Alert, selecting the Email template you just created

Step 5: Enable/Activate your workflow

Step 6: Update your Invoice record's Picklist value to one of the relevant picklist values and the workflow should fire (as a result of Step 3) sending an email (as a result of Step 4) in the format required (as a result of Step 2).

I haven't tested the above, as I've just typed it directly in here but this approach works as I have used it before.

For a low/no code solution, you could play around with something like this:

Step 1: Create a Picklist on your custom object (lets call this Template__c on object Invoice__c) which will be used to dictate the Email attachment content, e.g. Picklist values

• Email Template 1
• Email Template 2
• Email Template 3

Step 2: Create your Email template as a Visualforce template, only rendering the relevant content for your attachment, for example:

<messaging:emailTemplate subject="Invoice - {!relatedTo.Name}" recipientType="Contact" relatedToType="Invoice__c">
    <messaging:attachment renderAs="pdf" filename="{!relatedTo.Name} rendered="{!relatedTo.Template__c == 'Email Template 1'}">  
        <c:MyVFForTemplate1/>    
    </messaging:attachment>
    <messaging:attachment renderAs="pdf" filename="{!relatedTo.Name} rendered="{!relatedTo.Template__c == 'Email Template 2'}">    
        <c:MyVFForTemplate2/>    
    </messaging:attachment>
    <messaging:attachment renderAs="pdf" filename="{!relatedTo.Name} rendered="{!relatedTo.Template__c == 'Email Template 3'}">    
        <c:MyVFForTemplate3/>    
    </messaging:attachment>
    <messaging:htmlEmailBody >
        Dear Customer,
        
        Here is your Invoice.  Please pay now, or else!
    </messaging:htmlEmailBody>
</messaging:emailTemplate>

(note, i have illustrated the example using a VF component but you could just as well copy/paste the VF page content into the Email template.

Step 3: Create a workflow rule with the following attributes:

• Object = Invoice (Invoice__c)
• Evaluate the rule when a record is: created, and any time it’s edited to subsequently meet criteria
• Run this rule if the following criteria are met Template__c = 'Email Template 1' or 'Email Template 2' or 'Email Template 3'
• 'Save & Next'

Step 4: Add a new Workflow Action: New Email Alert, selecting the Email template you just created

Step 5: Enable/Activate your workflow

Step 6: Update your Invoice record's Picklist value to one of the relevant picklist values and the workflow should fire (as a result of Step 3) sending an email (as a result of Step 4) in the format required (as a result of Step 2).

I haven't tested the above, as I've just typed it directly in here but this approach works as I have used it before.