"Patches welcome" is an aggressive, user-hostile, anti-social response to being told that the thing you suggested does not work. It's telling the user to fuck off because your own suggestion was flawed. clarry didn't run to HN and scream "pdf.js sucks!".
"I'd rather bet on something you can implement all by yourself without needing to wade through a thousand page spec."
pdf.js was obviously outside the bounds of what he said he wanted from the beginning. You stubbornly brought it up anyway, then gave a nasty, clichéd response when he pointed out exactly why it was the wrong answer.
"If it doesn't work for you, fix it" is ridiculous. It's saying "here's this thing that doesn't do what you want, go make it do what you want instead of using these other things that already do what you want".
I think you're reading too much into that, or have a chip on your shoulder, or both. "Patches welcome" is an invitation, a smiling, friendly, we-think-you're-good-enough-and-want-your-help, open-handed gesture that is meant to encourage cooperation and evoke the deeply human drive to help others.
nknighthb is completely right. It is user hostile. It even makes a huge assumption, that clarry is programmer in the first place. With "Patches welcome" you might as well be saying "If you don't like it, spend a year learning [Language] to fix this issue." How is that anything other than user hostile?
If my mom can't open a pdf in pdf.js, I'm not going to tell her "Well mom, patches welcome."
Give that clarry is a) posting on HN and b) apparently at least contemplating writing his/her own OS at some point, the assumption that he/she is a programmer is anything but "huge".
But I try not to depend on too many things and people, if there's a way around it. I like to have control. That's freedom to me, and freedom gives me peace of mind.
So, no, I don't see why I should waste my precious free time improving software support for a format that I find way overcomplicated and just plain silly. Why should I?