If a pdf has too many images it might weight a lot, this is caused because generally (specially images created with a high resolution camera) could be significantly impacting in the size of the pdf.
Fortunately there is an easy solution, you can configure the plugin to compress the images before adding them to the pdf. To do so please follow these steps:
1.-Click in template settings
2.- Go to the Optimization tab
3.- Select an image compression level.
The higher the compression level the smaller the pdf generated will weight.