Use PdfReader and PdfMerger

[rongxin-liu]

No description provided.

[dmalan]

@rongxin-liu any chance this will speed up merging, which has been slower since our last (necessary) change?

[rongxin-liu]

@rongxin-liu any chance this will speed up merging, which has been slower since our last (necessary) change?

We used to do this for merging PDFs, but WeasyPrint broke at some point (method 1):

    if documents:
        pages = [page for document in documents for page in document.pages]
        documents[0].copy(pages).write_pdf(output)

So we switched to doing this (method 2):

    if documents:
        merger = PdfMerger()
        for document in documents:
            merger.append(PdfReader(io.BytesIO(document.write_pdf())))
        merger.write(output)

I'm unsure how this would impact the speed of merging, but the code owner from WeasyPrint did recommend we adopt method 2 and avoid using method 1 per:
Kozea/WeasyPrint#1662 (comment)

[rongxin-liu]

@dmalan I reverted our code to use method 1, since that method is faster and the generated PDF looks fine.