It appears to be a bug in pdf.js (or the PDF format itself): https://github.com/mozilla/pdf.js/issues/5064 https://github.com/mozilla/pdf.js/issues/5064 As som

It appears to be a bug in pdf.js (or the PDF format itself): https://github.com/mozilla/pdf.js/issues/5064 https://github.com/mozilla/pdf.js/issues/5064 As somebody else mentioned, you can use the arrow/triangle to display the regular viewer toolbar, and zoom out. This is obviously not ideal, but there's not much else that can be done until a fix comes from pdf.js upstream, I suppose.