If you're concerned about the time it takes to generate your PDF, using smaller image is a very good start. The smaller the images, the faster they are to load and thus, the faster your Document generation will be.
Our recommendation is to favor formats that provide small images. PDFMonkey supports recent image formats like Webp and AVIF. You can find free image converters online that can help you get smaller images without sacrificing quality.
Another good idea is using inlined SVG or data-uri images as they will not need to load. But even for those cases, optimizing the images will be beneficial.
If you wish to keep using more classic formats like JPEG or PNG, our recommendation is to use a tool like ImageOptim to get smaller images.