[kccqzy]

Maybe the letter you intend to add is not part of the subsetted font. Font subsetting is extremely common.

Maybe out of coincidence all the letters are present. Then you'd have to deal with manually adjusting the spaces and reflow the text. Reflowing the text can be done, but cumbersome. It's akin to fixing a bug in program not by changing the source and recompiling, but by binary patching.

In contrast, it's much easier to delete some letters in the PDF and keep everything else in the same place. In fact I've had obvious PDFs that have a copyright notice on every page. Deleting that can be done with qpdf and just vim (basically deleting the Tj or TJ operators).

This is fascinating. I recommend you read the PDF specification.