How do I change page size of an existing .PDF?

Currently we could change the page size of the PDF document by printing to Foxit Reader PDF Printer/Foxit PhantomPDF Printer with a designated Layout. Here are the procedures:

1. Open the PDF file that you want to change the page size.
2. Go to File > Print  to bring up the Print window. 
3. Choose Foxit PhantomPDF Printer/Foxit Reader PDF Printer, click on Properties, go to Layout tab, and choose a suitable page size, click on Ok to apply the change. 
4. Click on Ok to print. Then you'll get the same PDF document with a different page size.

If you're using Foxit PhantomPDF, you could also enlarge the page size by following the following:

Go to Organize > Crop Pages > double-click on the page to bring up the Set Page Dimensions window. In Change Page Size section, please choose fix size or custom per your request.

Relative Topics:

Optimizing PDF Documents for File Size

https://www.foxitsoftware.com/blog/optimizing-pdf-documents-for-file-size/

More Tips for Managing PDF File Size

https://www.foxitsoftware.com/blog/more-tips-for-managing-pdf-file-size/