> That usually tends to make collision attacks much harder, because you either have to make the resulting size the same too, or you have to be able to also edit the size field in the header.

> pdf's don't have that issue, they have a fixed header and you can fairly arbitrarily add silent data to the middle that just doesn't get shown.

This doesn't seem like much of an obstacle, since you can add silent data to all kinds of files, like

- With HTML, JS, etc. you can just add whitespace.

- Some formats like GIF89a have variable-length comments.

- With any media format that uses palettes, you can add extra, unused colors.

- Just about any compression algorithm can be tuned to manipulate the compressed size. E.g. with DEFLATE (which is used by PNG in addition to some archive formats), you can use a suboptimal static coding rather than the correct Huffman tree.

- With most human-readable document formats, you can add zero-width spaces or something similar.