Changing the version alone in the header of the file doesn't fix the issue. Looking on the net, the issue is with Adobe and something like a 200in limit. Using pdf version 1.6 and adding/modifying some UserUnit entry to 75,000 entry is supposed to fix it. Have to investigate more. If we can find a solution, perhaps Opera could add that automatically. But, ultimately, it's an Adobe issue.
Hmm, actually, the /UserInit has to be in the /Page section near the end of the file.
replace_all(src, "<</Type /Page\n", "<</Type /Page\n/UserUnit 72\n");
However, all that does is scale the font-size kind of. It doesn't get rid of the error in Adobe or anything like that. I tred a value of 75000, but that didn't do the trick. Tried 2 and 2.83 to see if that'd increase the allowed dimensions, but that again just increased the scale. There's something about /Mediabox dimensions, but I have know clue if those have to be modified too. I don't think so though.
So, I don't know. If I find any more info I'll post. For now, use a different reader.
Thank you very much for the efforts you've made for this issue. Indeed, if Opera can do this modification automatically, it would be most convenient for all users. There are too many technical things after all.