Flatten layers for PDF export for printing?

[kwrass]

I am using Designer. In preparing to export a book cover to PDF for printing, aren't I supposed to flatten all layers? I do not see an option to flatten layers before I export?  

For example, KDP lists instructions for uploading a book cover: "Flatten all layers, save the file as a press quality PDF with CMYK color profile, and then upload the file."

Am I missing something here?

[lacerto]

6 hours ago, kwrass said:

For example, KDP lists instructions for uploading a book cover: "Flatten all layers, save the file as a press quality PDF with CMYK color profile, and then upload the file."

In KDP instructions "flattening" typically means that there must be no unresolved transparencies in the document. The easiest method to produce all CMYK and transparency flattened PDF in Affinity apps is to export to PDF using PDF/X-1a (as it rasterizes transparent elements when they are flattened), but might also rasterize placed PDF content, so that an alternative would be to manually flatten elements containing transparencies on the canvas to optimize how flattening is done. That may well be unnecessary in your job or might make creation of the print file unnecessarily complex, so one way to make sure that the job will be exported in high quality is to make sure that the document DPI (as shown in File > Document Setup) is at least 300DPI.

Another thing that might be meant by "flattening" (besides transparency flattening) is that text elements are wanted to be in outlines instead of using fonts, and the easiest method to get that is to choose "Text as Curves" in the "Embed fonts" section of the "More" options of the PDF Export dialog box. But that should not be necessary if you export using PDF/X-1a since fonts will be embedded, and PDF/X-1a is often explicitly suggested as the recommended export method in KDP book cover instructions. 

One more thing: the citation above mentions "CMYK profile" which is a bit odd in KDP context. Is any specific profile mentioned? If not, I assume that you are using the default CMYK profile used in Affinity apps (US Web Coated v2), which should be just fine for KDP cover production.