How to Resize a PDF to A4 or Specified Paper Sizes?

Sometimes you may need to merge PDF files of various paper sizes. However, it will result in a merged PDF file with inconsistent paper size. You can use the following method to convert a PDF file into the same paper size as the other, and then merge them together. In this way, you can also avoid the printer from auto-selecting papers from different trays when printing.

1. Open your PDF file with Gaaiho Doc or any PDF readers, select "File" > "Print", and choose "Gaaiho PDF" virtual printer
2. Uncheck "Choose paper source by PDF page size" checkbox
3. Go to "Properties..."
4. Select the "Paper Size" you wish to convert to, and click "OK"

This way, the paper size of your newly created PDF file will be the size you specified.
Then merge this file with other PDF files.