Hello,
Sorry about the delayed answer!
I have not managed to compress images in a template string function, but I have managed to reduce image file size upon file/image creation. E.g when capturing images using your device camera, the image quality is set to a lower value than default(this is a setting on the create file object action node). By doing this, your PDF will as a resut be “smaller”.
Action node settings:
If your images are created elsewhere and imported, I would suggest compressing them before uploading them to the app or using a third party compression service to compress large images 