# Building my own UI doesn't work - no worker file

**URL:** https://pdfjs.community/t/building-my-own-ui-doesnt-work-no-worker-file/1507
**Category:** Technical Support
**Tags:** pdfjs-express
**Created:** [March 28, 2022, 8:28am UTC](https://pdfjs.community/t/building-my-own-ui-doesnt-work-no-worker-file/1507 "2022-03-28T08:28:55Z")
**Posts on this page:** 1
**Showing post:** 1

<div class="post-metadata">

### Author: ![dorota.k](https://avatars.discourse-cdn.com/v4/letter/d/e79b87/32.png) [@dorota.k](https://pdfjs.community/u/dorota.k)
#### Post date: [March 28, 2022, 8:28am UTC](https://pdfjs.community/t/building-my-own-ui-doesnt-work-no-worker-file/1507/1 "2022-03-28T08:28:55Z")

</div>

**Which product are you using?**

**PDF.js Express 8.3.0**

**Detailed description of issue**  
I want to create my own UI with @pdftron/pdfjs-express (8.3.0) and integrate it with React (17.0.2). I followed [this page](https://pdfjs.express/documentation/ui-customization/core), declared `webviewer-core.min.js` script in `index.html` template. When the script runs, an error appears. It says that there is no worker. In fact, the script needs to load `/pdf/PDFworker.js`, but there is no such a file in the library. I’ve found worker file like `/pdfjs/pdf.worker.js` and even tried to use it as PDFworker.js, but it made another error, so I may guess that file is incorrect. My question is: where to find the missing worker file and what exactly should it be?

I had used pdftron before and everything had been working great until I changed the library to PDF.js Express.

**Expected behaviour**  
Building my own UI with the library, but at least to load pdf file in a correct way.

**Does your issue happen with every document, or just one?**  
Every document

## **Link to document**

**Code snippet**  
I followed [this page](https://pdfjs.express/documentation/ui-customization/core) after changing library from pdftron.

---

_[View the full topic](https://pdfjs.community/t/building-my-own-ui-doesnt-work-no-worker-file/1507)._