Why does stretching happen?
When we convert your PDF to an online publication, we apply a fixed page size to each page.
The first page of your PDF determines the size of subsequent pages.
Stretching occurs when you have pages which use a different aspect ratio or orientation to this first page.

How to avoid stretching
To avoid stretching, upload a PDF with uniform page dimensions.

It is common with booklet-type documents to store the left and right pages as one connected spread. The converter recognises these spreads and will split them into two separate pages.