If there is acids and lignons in paper are what causes newspaper (and other aged papers) to yellow with afge. If your pages are not buffered, that acid could also leech into the non-acidic items and cause them to yellow.
It seems as thougth acids and lignons were added to make the paper process cheaper. That's why you may see very old items that are not yellow, but items frm teh 70s or 80s that have already yellowed. Some things will yellow after only 5 to 10 years.
Safest thing to do is to scan your non-safe items and print on safe paper with acid free inks.
I tend to just mount these items on buffered paper and used buffered pages to keep teh acid from spreading, being careful not to let tehse items touch any of my pictures.
Others have noted using and archival spray to encase tehse non-safe items.