So I did a little more research and it seems that you were able to select multiple files before the switch to iPhone because is a feature of the AndroidOS that is not supported in other mobile devices OSs.
However, if you are sharung your PDFs documents as email attachments follow this guidance:
you may try a similar procedure like the above thread when you're selecting multiple PDFs from the document cloud.
If you are trying to select multiple PDFs that are stored locally in your iPhone device see if you can install a file explorer from the appstore that allows the user to perform more file management function. Make it the default file explorer in your device so that your apps, like Acrobat Reader can take advantage of those features