embedPdf requires explicit image embedding when embedding a PDFDocument

[wistpotion]

I am writing a little website that (in short) imports images and then exports those as a printable and easily put together zine. Imported images can be either one page or a spread, so spreads have to be split into individual pages before they can be remapped for the final print.

Essentially what is happening is:
Import images ->
copy single pages and split spreads into single pages and put those into a PDFDocument where each page is an a5 (let's call this document split)->
remap pages and put remapped pages into a PDFDocument where each page is a4 (let's call this document final)->
turn the document into the blob that can be downloaded

The final document ended up outputting nothing when using the split document, but did output when importing a different document (https://pdf-lib.js.org/assets/with_large_page_count.pdf). Odd. After a while I tried .save():ing the split document and then embedding it into the final document, and then it worked fine. Doing the same but .embed():ing images in the split document and omitting the .save() procedure also worked just fine. That is not very intuitive, and was not (that I could see) documented anywhere. Maybe that should be looked over, so it's less easy to land in the same situation as I did.

Thanks for an amazing library! This is the first hiccup I've had with it, overall it's been great to use.