Append blanks at the end until the count divides by four.
Split the PDF into single pages, then merge them in the order 8-1, 2-7, 6-3, 4-5.
Turn off fit-to-page and multi-page. Fold after printing, not before.
Fold A4 in half and the sheet carries four A5 pages: 4 and 1 on the front, 2 and 3 on the back. Eight A5 pages become two sheets, printed as 8-1 / 2-7 outside and 6-3 / 4-5 inside. Get the pairing wrong and the whole booklet reads out of order after folding.
If it is not, the middle spread is missing and every sheet after it shifts by a page. Add blank pages at the end until the count divides by four — that is what the empty page at the back of most printed booklets is doing.
Duplex printers flip on the long edge by default, which suits portrait documents. A booklet has to flip on the short edge, otherwise the back side prints upside down. And do the page ordering yourself instead of trusting the driver's booklet mode: that mode scales pages to fit two per side, so the text comes out noticeably smaller. Order the pages, then print plain duplex at 100% actual size and fold by hand.
Yes. With a count that is not a multiple of four the inner spread is missing and every later sheet lands one page off.
Short edge for booklets and landscape content. Long edge is only correct for ordinary portrait documents.
A booklet mode or fit-to-page scaled it. Print at 100% actual size and let the fold do the shrinking.