SHA-1 – Why are text containers used for certificates?
The PDF or PostScript is used so that the collision is made very graphic. It is a public relation tool.

Known collision attacks on hash functions like MD5 and SHA-1 have some restrictions: the attacker can choose most of the colliding pair of messages, but the two blocks where interesting things occur are essentially random, and they will differ by only a few bits. The remainder of the colliding messages will be the same for both messages. So, as plain text, these blocks will be definitely non-pretty, and that's where the collision occurs. Thus, a plain ASCII text demonstration of the collision will not be "convincing" except for cryptographers.

Use of PostScript or PDF allows to leverage that small difference into arbitrarily spectacular differences; basically, the attacker can choose two completely different graphical contents for the whole page.

When the first collisions on MD4 were published, Dobbertin made the effort of making a sort-of ASCII text demonstration. His colliding pair consists in:

********************
CONTRACT

At the price of $176,495 Alf Blowfish sells his house to Ann Bonidea. ...


and:

********************
CONTRACT

At the price of $276,495 Alf Blowfish sells his house to Ann Bonidea. ...


in which the "*****" are twenty ugly-looking bytes that the attacker will claim to be there "for security reasons". MD4 is sufficiently broken that Dobbertin could make a demonstration with such mostly ASCII text, but he still had to include bytes that do not map to meaningful ASCII text.