I've downloaded InDesign 5.5 onto my Mac, as I need to use this version to work with some InDesign files created in this version. My Mac is on the latest Sierra system. But when I try to download it, I get this message"“Install” can’t be opened. You should eject the disk image." - Does anyone know how I could fix this? Thank you!
InDesign CS5.5 or CS6 is not supported under El Capitan or Sierra.
Presumably you have a later version of InDesign. Get your customer to save their InDesign document as an IDML file and see if you can work with that.