To deskew a pdf page, you can try the below three workarounds.
1. While creating pdf file from Foxit PDF Editor with your scanner, the option Deskew has been enabled by default as the below screenshot to deskew file automatically.
2. If you’d like to deskew the created pdf file, please open your file in Foxit, go to File>Optimize Scanned PDF, set the option Deskew to On to enable it, then click on OK.
3. Rotate manually
The scanned page is an image object normally, so we can rotate the whole page as image object. After open the scanned pdf file, go to Edit>Edit Object>Image, select the whole page, you will get the circular arrow to rotate the page as you need.
If you cannot see the circular arrow, please help to ensure that you are in the single mode.