Having used it as a key part of my own applications, I can confirm PDF.js is great. Works well even on lower-powered mobile devices.
Regarding Manager, I mostly upload PDFs as well, like others that have replied here.
Most of my expenses are for online businesses that send (or provide downloads) of the invoice, rather than me receiving it physically. Being able to upload the PDF file(s) directly on the edit screen would mean I don’t need to save and then attach the invoice separately.
However, this is ultimately a minor feature & the existing process of uploading it on the detail view instead does work. So I wouldn’t be devastated if support for PDFs here was a lower priority.