After a double-sided scan every back page came out upside down. How do I fix them all at once?

Why the backs in particular

A double-sided sheet is flipped either on its long edge, like a book, or on its short edge, like a top-bound notepad. Print it one way and scan it with the setting for the other, and every back comes out upside down. Because the scanner captures the front and then the back of each sheet, all the backs land on even pages: 2, 4, 6 and so on.

The steps

1. Open Rotate PDF pages and choose the scanned file.
2. Under 'Apply rotation to', choose 'Selected pages'.
3. Type the even pages into the pages field. Commas or spaces both work, so 2 4 6 8 is the same as 2, 4, 6, 8.
4. Press '180°' once.
5. Scroll the thumbnails: every page should now be upright. Then press 'Download rotated PDF'.

Why 180 rather than 90 twice?

The result is the same, since two presses of 'Right 90°' also make 180. But the '180°' button is one step and harder to get wrong, especially on a long file where you might press three times and end up with sideways pages.

When the pattern is not regular

• A single-sided sheet in the middle: its back is blank, and rotating it does no harm, but you may want it gone. Rotating does not delete pages, so use Delete PDF pages for that after rotating.
• Some backs were already fine: click their thumbnails to unselect them before pressing, or if you already pressed, select just those and press '180°' again to bring them back to 0.
• A very long file: typing the even pages one by one is tedious, but it is the right way. A range like 2-40 selects every page from 2 to 40, odd ones included, so it does not work here.

Next time, look in the scanner settings for the duplex flip direction and pick the one that matches how the pages were printed, so the backs come out upright from the start.