Looks like he did it "the hard way" --- and unfortunately it's not a truly valid PDF since the startxref isn't within the last 1KB of the file and the version number in the header is corrupt. Not all PDF readers will accept that.
On the other hand, it is possible to make a completely valid PDF and bootable ISO. The first 32KB of an ISO is officially "unused", which is probably why GRUB decided to put itself there, but that can be relocated somewhere else --- the El Torito boot descriptor will need to be updated to point to it --- and the PDF signature (which can be a valid one) and as many objects as will fit can be put in that area, with the rest anywhere else. The xref table can be moved to the very end and the offsets updated to point to the objects.
On the other hand, it is possible to make a completely valid PDF and bootable ISO. The first 32KB of an ISO is officially "unused", which is probably why GRUB decided to put itself there, but that can be relocated somewhere else --- the El Torito boot descriptor will need to be updated to point to it --- and the PDF signature (which can be a valid one) and as many objects as will fit can be put in that area, with the rest anywhere else. The xref table can be moved to the very end and the offsets updated to point to the objects.