If it was everyone could do it and my skills would be redundant. I've seen comments like that used and to be honest, it does bug me quite a bit. While the end result does require making a PDF to send to a printer there are other things a comic designer does.
Firstly you set your document up, check all the art pages to make sure they're the correct size, resolution, colour space (greyscale/CMYK) etc and adjust them if necessary. I even retouch artwork up, making sure the gutters are nice and clean and everything looks as good as possible.
Then all the pages are imported into the document, sized to fit.
Next stage is designing the covers, adding logos, text, barcodes etc, then any interior design work that needs doing (credit pages, adverts etc) all following the style established for the book(s).
When all this is done, then a proof PDF is sent to the writer/artists to check for any mistakes/amends that may need doing. Normally a couple of proofs are done before the book is 'signed off' for print.
I then create a print PDF file that is optimised for press, doing all the pre-press work to ensure that the book reproduces as I intend it to.
Luckily Stuart at UKcomics is a smart cookie and knows his stuff and tells me if there are any issues (very rarely are there any) and then it's a case of waiting for the printed books to arrive.
Hopefully you can see that there is a lot more than goes into this job than just 'making a PDF' the printed book is only as good as the files I send to the printer, as every piece of art is checked and adjusted by me before it goes to print, plus all the actual design work that goes into a book means that, often overlooked - a comic designer is one of the important cogs in the machine.