From Dorico Pro 3.5 To Affinity Publisher Tips Needed

I’ve just started working on a songbook in Affinity Publisher 1.10.1 (mac OS 11.5).

I’ll be importing multiple PDFs (exported from Dorico as monochrome), adding page numbers and front matter (as black-and-white text) and exporting a PDF to send to a commercial printer. It will contain no color (except for the cover - a separate file). My understanding is that print shops require PDFs in the CMYK colorspace.

I have no prior experience working with a print shop or DTP software.

I realize that I need to make sure the PDF exported from Publisher uses pure black (100% K), not rich (4-color) black. I know how to make the text added in Publisher pure black, but I’m not sure about the music PDFs.

Have any of you done this kind of thing before, and if so, can you explain the process?

On the Affinity forum, someone suggested I import the PDFs via “Add pages from file”, then do “select all”, and set the color to 100% K in the Color sidebar.

The one or two times I tried importing files this way, I think Publisher messed with some of the objects on the page. So I’d rather not use this method if I can avoid it.

I’d rather use PDF Passthrough if possible. It feels much safer!

But I don’t know whether Affinity truly leaves the music PDFs untouched upon export and, if it does, whether a print shop will accept Dorico’s grayscale output.

Thoughts, anyone?