You may notice they never do these stunts on ASCII text files.
The attack always requires to be able to store a large number of random data on the document.
There are 62 different bytes in the provided PDFs : part of these are the edited message, part of them are the 'attack'. That's 496 bits.
So it may be applied to any kind of document allowing to store about 496 bits of (invisible) data on it (and to be on the safe side, I'm assuming even half of that could be enough).
Not all successful attacks will require the size of the document to not change.
To look at it on a subject I dabble in (DNSSEC):
In DNS the chain of trust puts a hash of the zone key on the level above, along with a 16 bits checksum.
I'm assuming the checksum to only represent a 'minor' inconvenience.
This means that an RSA DNS key may be forged as it's basically stored as the exponent followed by any number of bytes (limited to 1KB : more than enough).
On another hand Elliptic curve seems out of the question as their size is small and constant.
All this to say, the vulnerability of SHA-1 is to be taken with a grain of salt.