The most efficient and fastest way to export multiple records from PubMed into Papers is to save them as a .nbib file and use the web version of Papers.
In PubMed, once you have located your publications, first click on the "Send To" button and choose "Citation Manager".
You will then have the choice of exporting all marked, the records on the page, or all records in the search. Choose which ones to export, then click on "Create File".
This will start a download of a new file with a .nbib extension.
Open Papers in your browser and click on the "Add" button. Select the "Add Files", choose the .nbib file, and then pick the desired library and list destination.
You may also find these articles helpful: