I have been active on the CreateSpace/KDP forums since 2009. I think the cover templates were developed around 2010 or 2011. I don't remember problems with the templates mentioned on the forums. As an aside, the KDP templates are now 600 dpi, and they have reduced the number of pages covered by each template from 10/11 to 6/5 (as I recall). Bookow templates (https://bookow.com/resources.php) are 300 dpi and are made to fit each trim size.
I should say that I always make my PDFs from a JPG. I do this because I have seen PDFs open showing, for a second, prior states for material that was erased, changed, etc. Anything I can do to keep the current state pure and clean, has to be a positive assist when entering the print-on-demand environment.
Separate from this post or the question at hand, KDP Technical Support (therefore Customer Support) claims to have proprietary software to test for transparency, I suspect they use Acrobat. Technical Support claims that JPGs can have transparency (click on an image in a PDF, the blue overlays shows transparency), and that the transparency grid (Preferences > Page Display > Show transparency gird) also shows transparency, although, ironically, it does not show the transparency in JPGs. Whether they are delusional or not, they can block a book for having transparency, and it is impossible to prove otherwise (I've tried).
Also separate from this post, while IngramSpark asks for CMYK and PDF/X-1a or PDF/X-3, KDP will accept almost anything. It consistently prints a tad better (in terms of color fidelity and shadow detail) from sRGB than from CMYK.
Back to the question of transparency, while KDP can find transparency in a ham sandwich, if Acrobat clearly identifies transparency in GIMP produced PDFs, that is asking for problems. How often does this happen? I don't know. All blocking issues come and go in waves, probably related to training seminars (or the equivalent), with each lasting about a 6 months to a year. Not everyone with a problem finds their way to a forum.
I am hoping for a practical way for people, who use GIMP, to generate a PDF without transparency.
I should say that I always make my PDFs from a JPG. I do this because I have seen PDFs open showing, for a second, prior states for material that was erased, changed, etc. Anything I can do to keep the current state pure and clean, has to be a positive assist when entering the print-on-demand environment.
Separate from this post or the question at hand, KDP Technical Support (therefore Customer Support) claims to have proprietary software to test for transparency, I suspect they use Acrobat. Technical Support claims that JPGs can have transparency (click on an image in a PDF, the blue overlays shows transparency), and that the transparency grid (Preferences > Page Display > Show transparency gird) also shows transparency, although, ironically, it does not show the transparency in JPGs. Whether they are delusional or not, they can block a book for having transparency, and it is impossible to prove otherwise (I've tried).
Also separate from this post, while IngramSpark asks for CMYK and PDF/X-1a or PDF/X-3, KDP will accept almost anything. It consistently prints a tad better (in terms of color fidelity and shadow detail) from sRGB than from CMYK.
Back to the question of transparency, while KDP can find transparency in a ham sandwich, if Acrobat clearly identifies transparency in GIMP produced PDFs, that is asking for problems. How often does this happen? I don't know. All blocking issues come and go in waves, probably related to training seminars (or the equivalent), with each lasting about a 6 months to a year. Not everyone with a problem finds their way to a forum.
I am hoping for a practical way for people, who use GIMP, to generate a PDF without transparency.