Hello and thank you for submitting this question.
Cropping pages in existing PDF document is easy done.
Once the file if is opened with Expert PDF , check Page size in Document properties in case is unknown, then go to Edit tab on the toolbar and click on the arrow underneath Size options and select Page size setup use the fixed option of set custom Width and Height.
Once the page size is set, and you need to shift the content so to fit the page, please zoom out to around 30% and press key combination Ctrl+A to select all content on the page (or use the Edit mode in Edit tab to select single object) and correct its position.
Save the document as new PDF file or overwrite the existing one to keep all made changes.
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