Altium Designer: How do I scale the output gerber file generated by Smart PDF into 1:1 ratio?

File -> SmartPDF is not the route you require to get your output on paper in 1:1 scale.

SmartPDF is intended as a documentation output for review purposes, not a fabrication output.

To create a fabrication output you need to create an output job file.

In your PCB project, Right click the project, add a new output job file. Edit the output job, add a new 'Fabrication output' Right click the new fabrication and 'Page Setup' set 'Scaled print' scale = 1.0 Link that to a printer and paper settings you require by drag and drop. Save the job, right click it in the project and do 'Produce output' That should then produce output on paper for your given printer at 1:1 scale.

You can create a PDF output (e.g. using pdfForge printer driver) rather than a real printer, but obviously that depends on the settings in the PDF reader and printer the person printing your PDF file.