Get images for PDFs from URLs
The main issue I see with this code is in the fetchFilesAsImages() function. You're needlessly building a results array to return at the end of the call fetchFilesAsImages(). Why not just take advantage of the fact that Promise.all() will already return a list of what each promise resolved to. So, you could instead write fetchFilesAsImages() as follows:
export async function fetchFilesAsImages(filesUrls) {
  return await Promise.all(
    filesUrls.map(async ({ url, category }) =&gt; {
      const data = await fetchBlob(url);
      return await convertPDF({ data }, category);
    })
  );
}

One other thing I would suggest is to just use async/await syntax everywhere. Your fetchBlob() could look a little cleaner with it:
export async function fetchBlob(url, opts = {}) {
  const res = await fetch(url, opts);
  if (!res.ok) throw new Error(res.statusText);
  return res.blob();
}