Keep in mind that PDF is not a text editor, word processor, or page layout application. Only minor editing can be done. Any significant format or layout changes is not possible.
An option. Export or Save As to another file format. Open that with a word processor or text editor application. Use that application's page setup to establish desired page size. Output to a new PDF.
Or, if the PDF contains a scanned image; export the image. Open in an image editor, edit as appropriate, use this application's page setup to obtain desired page size. Output to a new PDF.
Only minor editing can be done. Any significant format or layout changes is not possible.
An option.
Export or Save As to another file format.
Open that with a word processor or text editor application.
Use that application's page setup to establish desired page size.
Output to a new PDF.
Or, if the PDF contains a scanned image; export the image.
Open in an image editor, edit as appropriate, use this application's page setup to obtain desired page size.
Output to a new PDF.
Be well...
Be well...