Loading & Zoom of PDF takes several minutes

The following error File needs a long time to load after zoomed-in → it is unusable…every other library can handle it without any problem (pdf.js loads it normally).

Error File

Please check this file and the problem the library has with the loading this kind of file plus the zooming!


Thank you for the report and for sharing the file with us.

In the future, please fill out the issue template correctly or else your ticket will be closed.

Which version of PDF.js Express are you using? The file loads relatively fine for me in 8.1.0. It is a big sluggish zooming, but that is expected as this is a huge complex file.

Since this is an isolated incident and the file loads file in 8.1.0, there is not a lot I can do here.


Hi Logan,

I am sorry, haven’t seen the template. Next time will use it!

We are using the latest version 8.1.1 - it works faster using the Google Chrome browser, but using Firefox on low-end hardware takes up to several minutes if you zoom into the file. We were hoping that the PDF.js Express library would have the same real-time zooming & loading performance as the “original” PDFtron viewer?

Also, we have discovered a second bug - should I open a new ticket for this? This bug is related to the wrong Encoding of text inside the File. If you zoom in, the text is not readable, due to strange letters (the original text was in German, if you open it with Adobe, you see it correctly).

Would be great if you could look into this as well.



PDF.js Express will never have the same performance as PDFTron WebViewer - they use completely different engines. In this case, there is not a lot we can do with performance of files like this since the bottleneck is the typically PDF.js Engine we use, which we do not support.

If you need high performance for files like this, I recommend looking into upgrading to PDFTron WebViewer.

As for your other bug, yes, please open a new ticket.


Hi Logan,

so it means that PDF.js Express is based on PDF.js and not on the Engine which PDFTron is using?
But using the original PDF.js viewer from Firefox itself loads this file faster as well as the current PDF.js Express version.

Link to the other bug: Wrong encoding inside PDF

Yes that is correct, Express does not use the PDFTron engine.

I will add it to my backlog to look into the performance of this file, but I cannot guarantee that we will fix it.


Ok, thank you. It would be great if some improvement would be possible to increase the performance a bit.


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