This isn't about security, it's about making sure that a text document is formatted exactly the same on all devices, whether it's a PC, a phone or a printer.

The PDF file is reliable in that aspect while other widely used document formats like .doc and .docx are basically a gamble, even if you open the document on the same machine with the same software. The same with presentations: You just need to open the file in a slightly different software or version or encounter one of the countless bugs and suddenly you have a picture overlapping with text.

By your argument, PDF fails. I can provide examples of PDFs that look different on different devices.

PDF gives you all the tools to make sure that doesn't happen. No one prevents you from shooting yourself in the foot by writing a bad PDF that doesn't adhere to common good practices. For example it is technically possible to make a PDF with text without embedding fonts. Doing that is a bad idea.

By your argument, any programming language, no much how safe it purports to be, fails because they allow you to write bugs.

This was about the claim that PDFs provide consistent/identical document representation on every device.

Few programming languages claim to provide bug free programs, so your example is irrelevant.

> This isn't about security

kccqzy's comment was clearly referring to security.