How to query all files from a document library and send it as an attachment

I haven't done this before but it sure looks possible using the Send notification action. You can use the Add Attachment" link when configuring it. The help file gives this bit of information: "Allows file attachments to be added to the notification. You can specify multiple URLs by separating each URL with a semicolon. You can also specify multiple URLs by referencing a collection variable."

When querying a document library I believe you can directly get the URL of the documents into a collection variable and then use that in the Send notification. Worst case scenario is that you can build the URL of each document and concatenate them into into a multiple lines of text variable or build a collection and then use that variable in the Send notification.