memes

Actually, PDF is a turing complete programming language.

PDF is a simplification and wrapper around the computer language PostScript - a PostScript or PDF doc literally runs on the printer or computer and outputs the rasterisation of the thing you want to print.

PostScript is language based around a stack. You can define functions (which may be fully recursive) that run on the stack.

Here's a small example:

/ANGLE {
   newpath
   100 0 moveto
   0 0 lineto
   100 50 lineto
   stroke
} def

10 setlinewidth
0 setlinejoin
100 200 translate
ANGLE

1 setlinejoin
0 70 translate
ANGLE

2 setlinejoin
0 70 translate
ANGLE

As such, PDF that's actually similar to Python, and HTML is closer to something like a JSON or XML document.

Note however that HTML can contain Javscript or WASM programs, but these are embedded rather than features of HTML.