Earlier quoted context omitted.
I got what you're saying the first time, and you still seem to be entirely missing the point. Immutability means that an object cannot be modified after it's created, and any changes result in a new object rather than altering the original. You're saying "well, look, I can modify this pdf and I can even undo my changes...", what I'm saying is that whenever you modify a PDF, you're essentially creating a new file rath…
The people who act as if PDFs are legally immutable are not performing MD5 comparisons. Also, that isnt even an intention of the file format as far as I can see, its mostly a byproduct of cruft and backwards compatibility. No one would call .doc immutable because its very difficult to move an image and then restore that image to the original location. In this context, people will save something out as pdf and store i…
Immutability doesn't mean that an "object cannot be modified", it means that in order to modify an object, you must create a new (clone) object. That's all what I meant to say. Sure, you can get pedantic or otherwise and say "yes, pdfs are immutable; or no, pdfs aren't immutable in some contexts", etc., and depending on the point of view, both of these claims could be correct — I'm not arguing about the specifics.
I'm just saying that your explanation of why you think pdfs are not immutable hinges on an incorrect idea of what immutability actually is.
There's a rigorous definition for "immutability" in computer science, e.g., strings in many programming languages are immutable, but that doesn't mean you can't manipulate them, it just means that operations that appear to modify strings actually create new string objects.
The greatest illustration for immutability is imbued in programming languages with immutability-by-default, e.g., Clojure. Once someone groks the basics, it becomes really clear what that thing is about.