Write the pages you want as 1-3,5,8-. A dash after a number means to the end. Out-of-range pages and typos should be rejected with a clear message rather than guessed.
Taking pages 2-3 gives one new PDF. If every range should be its own file, such as 1-3,4-6,7-, you get one file per range, packaged in a ZIP so you download once.
To cut a long document into equal parts, use every N pages. For example, a 40-page file with N=10 gives four files.
Splitting makes new files and leaves the original untouched. Check the first and last page of each part before you send them on.
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