Easy way to change pdf document page size from US Letter to A4
Please refer this KB doc https://helpx.adobe.com/acrobat/kb/scale-or-resize-printed-pages.html and check if this helps.
Let us know if you need any further help.
Under edit PDF find the crop tool. Draw a box around your page, doesn't matter what size, and hit enter. In the dialog box that opens on the bottom left is Change Page Size. Select custom and change the page to the size you desire.
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