One page came out sideways in my merged PDF. Did the merge do that?

If it was stored sideways, why did it look fine before?

That is the confusing part, and there are three common reasons.

• You rotated the view only: many readers have a button that turns the page on screen without saving anything to the file. Close the reader and the file is as it was.
• The page was scanned sideways: the scanner or camera app saved the image on its side, and you had been reading the file rotated without noticing.
• It is a genuinely landscape page: a table or chart designed wide, which is exactly right but looks odd among portrait pages.

In the third case there is nothing to fix. A landscape page stays landscape because the tool respects every page's size, which is usually what you want for a wide table.

The fix: rotate the page in the merged file

1. Open the merged file in Rotate PDF pages.
2. Choose 'Selected pages' and select the sideways page, or type its number.
3. Press 'Right 90°' or 'Left 90°' as needed and check the thumbnail.
4. Press 'Download rotated PDF'. This time the orientation is saved in the file, so it looks right in any reader.

Should I fix the original before merging instead?

If you will use the original again, in a monthly merge for example, yes: rotate it once and save it, so the problem does not come back in every merged file. For a one-off merge, rotating the page in the result is quicker because you deal with a single file. Either way, rotation does not redraw the page content, so quality does not drop.

Worth knowing: if you rotate a page in the original and then merge, Merge PDFs carries the new rotation across as it is, because it copies each page's rotation value exactly as it finds it.