Most likely, you do not have the Android version of the app installed on your Chromebook. Unlike a phone or tablet, the Android version of Google Drive does not come pre-installed on Chromebooks. Instead, the Chrome app for Google Drive is pre-installed on Chromebooks.
To get Google Drive or any other app to show up as an option when importing a PDF into Squid, you must install the Android app through the Google Play Store app on your Chromebook. You can search the Play Store or use the links below for common apps:
For a video tutorial about importing PDFs, see Squid - Working with PDFs.