PDFs attaching in wrong order?

Hello!  I'm very new to Adobe--assigned to a work project to create a MSDS online document with pdfs of all the labels and sds sheets for chemicals we use.  I'm downloading the labels and sheets  to Adobe just fine but when I try to add each to my document, they are all scrambled in page order. 
I can't figure out why this is happening and how to get it to stop!  
many help is much appreciated!!!