Your duplex scanner scanned the front of every page into one file and the back of every page into another — then helpfully interleaved them in the wrong order. Now you have one PDF with odd pages reading 'front 1, back 1, front 2, back 2' which makes no sense.
Splitting into odd and even files is step one of fixing this. Flint does it in two clicks.
What 'split odd/even' actually produces
Open split PDF, choose 'odd/even split'. Flint produces two files: 'odd-pages.pdf' contains pages 1, 3, 5, 7 of the source. 'even-pages.pdf' contains pages 2, 4, 6, 8. Each output is its own ordered PDF.
Why this matters for duplex scans
Some scanners scan one side of every page first (all the fronts), then the other side (all the backs). The output is a PDF where front pages come first and back pages come last. Splitting odd/even doesn't help that directly. The case where odd/even split DOES help is when the scanner alternates correctly but the back pages are in reverse order — split, reverse one half, merge.
Printer workflows
Some old printers do duplex by printing all odd pages, asking you to flip the stack, then printing all evens. If your printer driver supports this, split into odd/even files, print odd, flip, print even. Modern duplex printers handle this automatically — only use the manual flow if your driver is misbehaving.
Recombining after fixes
Once you've fixed whatever was wrong (rotation, ordering), merge the two files back. The interleave order on merge controls how they recombine — drop them in alternating, or reorder pages for fine control.
FAQ
Does this work with non-numeric page counts?
Yes — odd/even is about page index, not page number labels. Page 1 of a Roman-numbered preface is still 'odd'.
What if my document has an odd total page count?
Odd file has one more page than even. Standard.
Can I interleave back differently than original?
Use reorder PDF on the merged result to set custom page order.
Useful for two-sided printing on a single-sided printer?
Yes. Print odd, flip the stack, feed back in, print even. Order of pages in odd/even files should match your printer's flip behaviour.
Two files, odd and even. Split odd/even pages.