[mrb]

Weird. I could swear "-flatten" didn't behave like this years ago when I last used my script. But maybe I am misremember...

Edit: haha! The "-flatten" needs to be replaced with "-alpha flatten". This way, multi-page documents are still handled correctly, and alpha transparency is also handled correctly. I just tried on this sample file with transparent images: https://tcpdf.org/files/examples/example_042.pdf

[ca7]

Changing "-flatten" to "-alpha flatten" (without the double quotes) results in an error for me.

> convert: UnrecognizedAlphaChannelOption `flatten' @ error/convert.c/ConvertImageCommand/673.

[mrb]

Probably due to an older ImageMagick version. Try "-alpha remove" which should be more or less equivalent (I think)