And it's a bad format, because .tar does not have an index that allows random access. It also does not have compression, unless you stick it in gzip, which makes it even worse because then you can't even skip entries, you have to decompress the whole thing from the top.

.zip is better, at least that has an index, but at the end of the file, so it's not linearly streamable.

For what it's worth, .pdf is structurally ~the same as .zip; the index is at the end, and blocks are compressed individually.

Choice of archive format matters :-)