Hello friends, I thought I'll share a few tips on how to upload multiple docs under each category of online visa application. I followed the same but I am yet awaiting the results of my application 
After trying many tools to merge multiple PDFs and facing issues in all, I came accross this great tool called PDF Architect 2, you can download it from pdfarchitect.org/download. The free version allows to merge multiple PDF files and then you can save the merged file on your drive and then uplaod it in the respective category. E.g. Proof of Income, I merged last 4 months pay stubs and 2 years W2 into one file and uploaded the same, just need to make sure the merged size is less than 4mb coz that is the limit on CIC site.
Hope this is helpful!
After trying many tools to merge multiple PDFs and facing issues in all, I came accross this great tool called PDF Architect 2, you can download it from pdfarchitect.org/download. The free version allows to merge multiple PDF files and then you can save the merged file on your drive and then uplaod it in the respective category. E.g. Proof of Income, I merged last 4 months pay stubs and 2 years W2 into one file and uploaded the same, just need to make sure the merged size is less than 4mb coz that is the limit on CIC site.
Hope this is helpful!