Black text becomes grey sometimes

[medwds]

I create adverts for a local newspaper using Apple Pages. These export in CMYK with black made up of the primary colours. To try to make things nicer for the printer, if the advert is to be on a black-and-white page I use the "Digital printing (B/W)" preflight profile in Acrobat, which normally changes the text to 100% process black and removes the CMYK plates.

In the latest advert I've created, the text becomes 94% process black (according to Output Preview). The advert includes a PNG logo with the D50 profile; if I remove this before exporting from Pages and then run the preflight profile, the text becomes 100% process black as expected. But if the PNG is present in the PDF, Acrobat makes the text 94% process black. Why might this be, and how can I have the text become 100% process black?

I attach examples of Pages' PDF export both with and without the PNG, along with the PNG itself.

[medwds · Accepted solution]

It turns out the PNG included a transparency (alpha) channel. Removing this solved the problem, and was an acceptable solution in this case because transparency was not needed.