No space between words when annotating some PDFs

### Steps to reproduce
1. Open PDF https://github.com/mozilla/pdf.js/files/2955999/1.pdf in the browser (Firefox or Chrome).
2. Launch the Hypothes.is client.
3. Select a few words to annotate, either in the middle of a line, or from one line to the next

### Expected behaviour
Spaces between words should be observed in the annotation excerpt. For example: "Open Sans is a humanist sans serif typeface"

### Actual behaviour
Instead, spaces between words are lost: "OpenSansisahumanistsansseriftypeface"

### Browser/system information
This is happening both with Firefox native's PDF viewer and Hypothesis' bookmarklet, and with Chrome's Hypothesis extension.

### Additional details
This seems to be a well known issue with PDF.js. See issues https://github.com/mozilla/pdf.js/issues/10640, https://github.com/mozilla/pdf.js/issues/6657, https://github.com/mozilla/pdf.js/issues/10110.

I have found several PDFs like this. Cases where the space at the end of a line is lost seem more common (https://github.com/mozilla/pdf.js/issues/10110).