Wraps FPDF_CreateNewDocument. The returned pdfium_doc has
no pages — add some with pdf_page_new() before saving.
Always returned with readwrite = TRUE; there is no read-only
new document.
See also
pdf_page_new() to add a page;
pdf_save() to persist the result.
Examples
doc <- pdf_doc_new()
pdf_page_new(doc, 1, 612, 792)
#> <pdfium_page [open] page 1 of <new>>
tmp <- tempfile(fileext = ".pdf")
pdf_save(doc, tmp)
pdf_doc_close(doc)