If you have several different PDFs that you'd like to combine into one document, you'll need to do combine them before you upload them to issuu, as you cannot create the document on issuu.
You will not be able to combine separate PDFs after you have uploaded them to issuu, as each of them would be counted as an individual publication.
Since everyone uses different tools to make their PDFs and those tools are not a part of issuu, we've found that this article provides helpful tips on merging PDFs, using different tools.
NOTE: If you're using InDesign to create your publication, you can download this issuu Export Preset and add it to your presets. This will ensure that during the export process your PDF is issuu-ready.