The maximum MB size of your file you can upload depends on the plan you currently are subscribed to.
A free plan can upload and convert files up to 25mb in size.
A Pro plan can upload and convert files up to 250mb in size. If you require a larger file size, we can make arrangements to support up to 2gb.
You can, however, start with one file and once it's been converted into a simplebooklet, upload a second file from the PAGE CONTENT tab under PAGES > INSERT PDF