TIL: PDF JS Preview

What is a TIL?

Today I learnt how to use PDF.js to add a PDF preview to a HTML form without having to upload the file to a server.

I needed this to build a multi-step form that shows a preview before we have reached the last step in the form submission.

A demo is worth a thousand words:

See the Pen PDF preview form by Tim (@betatim) on CodePen.

Check out the code of the codepen to see how it is done.

This is a useful little snippet of JavaScript that means users do not have to upload the PDF to your servers first to get a preview image. In this case the API that receives the PDF does not offer an option to render only the first page. You have to upload the whole document, basically submitting the form, and then tidy up again when the user decides not to submit the form.

A advantage of doing the rendering on the client side is that you do not have to upload the PDF first. This can be a huge benefit if your users are on a slow internet connection and have large files.

A drawback of using PDF.js is that if your PDF is expensive to render, it can take a while to get that preview image on a low powered device like a mobile phone.

One thing that took me a while to figure out is that you should not include the pdf.worker.js via a <script> tag. Instead you should set pdfjsLib.GlobalWorkerOptions.workerSrc = "https://unpkg.com/pdfjs-dist/build/pdf.worker.js" in your JS. Figuring this out took a while because a lot of examples either load it in <script> tag or do not make any mention of it. If you load it the wrong way you will see a warning in your console about PDF.js using a "dummy worker".

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