I think what you're seeing is the PDF Title, which is stored as metadata in the file. One of our customers appears to set this to "BFOEncoder output" in documents they generate with our software - that's their choice, it's not something we set.
I expect that if you load a document with a different title, you'd see a different value. For example, this one https://publisher.bfo.com/samples/poster/output.pdf is one of our sample documents, the title is "BFO Publisher Layout Festival" rather than "BFOEncoder Output".
Not every PDF sets the Title metadata so it won't happen with every file, which is why it may not have been noticed before now. And if for some reason it's happening with files that don't have this metadata set... well, in that case I don't know, but I promise it's nothing to do with us. I don't run Opera myself, I'm only here because my ears were burning 
Cheers... Mike (CTO at bfo.com)