It is not just about searching offline.
The fact that paperpile does not support full text searching of PDFs is a huge disadvantage on its own.
A responce I got in the past that we are supposed to use standard Google Drive search functionality for this purpose. However, this is extremely limiting, since Google Drive does not support restriction of search to a specific folder, does not differentiate search results between title/abstract/content of the article, and does not highlight search results in the text of the PDFs.
A workaround I came up with is to have my Paperpile Google Drive folder containing PDFs synchronized to my computer, and tell Mendeley to watch that folder. I can then use Mendeley for offline work and for searching the content, and use Paperpile as the importer and Google Docs citation manager.
Really look forward to either Mendeley providing integration with Google Docs, or Paperpile adding offline and search features, so I can use just one product. Or maybe someone else will come up with yet another reference manager that combines the best from these two products.