This is by design and not surprising at all if you read even a tiny bit about PDF. It's in fact the default save method in nearly every PDF capable software. Rewriting the PDF is in fact the less common method. I'm surprised a researcher of PDF would be surprised by that.
However, if you are using a tool like a redaction tool then the software should forbid you from writing in append mode. This was a common error in old PDF apps and perhaps contemporary ones that are new.
Edit for politeness:
My surprise is aimed at the researcher, not you :)
1. the one or ones being addressed
2. ONE sense 2a (which is “being one in particular”)
So, pro tip: in chat-like discussions with strangers such as hacker news, one should prefer saying “one” when using sense 2, even if it sounds a bit archaic (at least to me. Is it?)
Also, when reading a “you” that could be interpreted both ways, do not assume it is used in sense 1.
However, if you are using a tool like a redaction tool then the software should forbid you from writing in append mode. This was a common error in old PDF apps and perhaps contemporary ones that are new.
Edit for politeness:
My surprise is aimed at the researcher, not you :)