Sumatra PDF might be a bit more popular than Kayako. We know about 1 link to it since March 2021 and only 1 link to Kayako. We are tracking product recommendations and mentions on various public social media platforms and blogs. They can help you identify which product is more popular and what people think of it.
One we used at a previous place, Https://kayako.com/ Small company, I'm sure there was not much effort put in customizing it. No idea on pricing but wouldn't expect it to be on the high side. Source: over 2 years ago
You can use your browser for both, there are extensions(FireFox - PDF & EPUB/Chrome - PDF& EPUB) for that. In case, you want you use a separate software for them, I suggest SumatraPDF--it handles both PDFs and .EPUB files. Source: over 1 year ago
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