From the example in bug 73125:
• File:BathsOfCaracalla.jpg Source: http://upload.wikimedia.org/wikipedia/commons/c/ce/BathsOfCaracalla.jpg License: CC-BY-SA-
3.0 Contributors: ? Original artist: ?
• File:DomusAurea.jpg Source: http://upload.wikimedia.org/wikipedia/commons/c/c9/DomusAurea.jpg License: Public domain Contrib-
utors: ? Original artist: ?
So in the first case we know that attribution is required (BY) and that we're not giving it, but we include the image nonetheless. We should instead skip it (optionally leaving some placeholder/note).
In short: when you'd display "Contributors: ? Original artist: ?" in the PDF, and the file is not PD (as marked by [[ https://www.mediawiki.org/wiki/Extension:CommonsMetadata#Returned_data | AttributionRequired ]]), skip the file.