If you use any Premium elements, your low resolution PDF will definitely have watermarks - a criss-cross pattern superimposed on top of images to protect the element’s copyright holder.
However, these watermarks should already be gone when you generate and check your high resolution PDF on Step 3 of the Ordering Process.
If your PDF has the Canva watermarks, first, make sure that you're viewing the high resolution PDF. If you're sure that you're viewing the high resolution PDF and the watermarks are there, please take note of them and request they be fixed as touch ups on Step 3 of the Ordering Process.
On the other hand, if your page has non-Canva watermarks such as this:
Please replace them with a non-watermarked version. Unfortunately, we cannot fix them from our side because these photos were obtained outside Fusion/Canva. You might have to pay the image provider, where you got the photo from, in order to get the high resolution and non-watermarked version.