My workaround, and I realize this might not work for most people, is to copy the papers I need to search onto my laptop or cloud storage, tag the papers with a project specific tag and do the search from there. This does work from multiple different laptops and computers, but it doesn’t allow a search on files that are in Paperpile but not in a folder that is obviously related to my current project.
If you have an exact phrase you are searching for Google Scholar can sometimes be helpful.
I agree this is an important feature, and would very much like to have it.