Hello bernard meunier,
The PDF file format is coordinate based - so any images will be placed at given coordinates and will have given dimensions in the coordinate system of the page, so the DPI will be a result of those dimensions and the available number of pixels in the file. So to "change" the DPI of an image - you will need to resize it - you can do so e.g. by resizing the page it is on (as I understand you want to do that as well) - and select to scale the content as well. So please do give the "Resize Pages" tool a try and make sure to click on the "All Options..." button on the left - to turn on the content scaling too.
However given the coordinate nature of PDF files - I would recommend against resizing the pages now, and instead use 200% scaling when printing them to paper unless e.g. another system can't do that - and needs to get the files at the "right" dimensions.