Have you ever used Acrobat? Not "Acrobat Reader", but regular Acrobat, the most popular PDF editor. It's from Adobe, and it definitely does not "resist" edits.
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 rather than truly "undoing" changes in the original. PDFs have complex internal structures with metadata, object references, and possibly compression that make bit-perfect restoration challenging.
Unlike plain text files where changes can be precisely tracked and reversed at the character level, PDFs don't easily support this kind of granular reversibility. Even "undoing" in PDF editors often means generating yet another variant rather than returning to the exact binary state of the original.
Take a look at how Git stores PDFs - when the delta approach doesn't work efficiently since even small logical changes can result in significantly different binary files with completely different checksums, it stores EVERY version of the same document in a separate blob object.
When you annotate a pdf and then later change your mind, undo all the annotations and save it — only to your eyes it may look the same as the original — in digital reality, it will be a different file.
I feel like I'm talking to a toddler, sigh. Let me try again.
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.
