Which folder is root folder for mapped domain to put additional files?

I recently moved by merging a site from older host into multisite installation at wpengine. I adopted the traditional import/export wordpress way. Now, Earlier i was having a folder in the root directory which contained some files, which was obviously not imported. So, I contacted wpengine but the support staff was not able to tell me the location of directory where that folder can be manually created and those files can be added.
Example: earlier(standalone) I was able to get the site by url domain.com/PDF/file1.pdf
Now I want to know the folder (mapped multisite) where I can put the PDF folder with all files