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Sometimes you may want to extract only a few pages from a PDF, or reorder pages, or insert new blank pages. This is easy enough to do on a "real" computer using some tool like
pdfjam (wrapping the
pdfpages package for LaTeX) or
mutool or many others, but it seems somewhat harder on phones. No matter: for many of us, every operating system these days seems to be used primarily to launch another cross-platform operating system: the web browser.
To use this tool, drop a PDF file above, enter a specification of what pages you want extracted, then click on "Done". Then you can click on the link to download a new PDF file with reordered pages.
To get a single page, use the page number, e.g. for page 3 use "3".
A range of pages can be represented by using a hyphen (the ASCII stand-in for a dash), e.g. "2-4" means pages 2, 3, and 4. As a shortcut, one or both ends of the range can be omitted: "2-" means pages 2-n, and "-2" means pages "1-2". If in a range the latter number is smaller, then it means the pages in reverse order, e.g. "4-2" means pages 4, 3, and 2.
A blank page (that you want to insert into the PDF) may be specified by the digit "0".
A sequence of ranges can be specified using commas, e.g. "1-3,2-4" means pages 1, 2, 3, 2, 3, 4 (in that order).
Spaces are ignored.
(At least, everything above is what I intended, but the code may have bugs. Try it and let me know.)
The code rearranges pages using pdf-lib (GitHub) (v1.3.0) by Andrew Dillon. I tried to use Mozilla's PDF.js, but its documentation on these “core” matters (as opposed to just rendering a PDF in a canvas or whatever) seems poor/nonexistent compared to pdf-lib, which is excellent.
This page borrows heavily from something I made for a more specialized use-case: Möbius-printing helper.