You can add images directly to invoices; those will be saved in the invoice itself.
To learn more about how to add an image take a look here.
Other document types, such as PDF documents, XLS sheets or different document types can be sent with an email as attached files.
iOS MOBILE APP
Select a document and click on Email
A new window with the iOS Mail App pops up
Click on the email message (or hold your finger for 1 second on older iOS versions)
A menu pops up
Select the arrows to Add attachment, Add document or Insert photo or video
Locate the desired attachment to attach it to your email message
Learn more about the process on Apple's user guide
Click on the Email icon to send the chosen document
A new window Send by mail pops up
Click on Choose file
Locate the attachment you'd like to add to your email message
You can add any files with a max. size up to 5MB (together)