Is subscribing to Squid Premium the only way to get the premium features?
No, you can still purchase the PDF Import, Tool Pack, and Cloud Backup premium features individually (as in-app purchases within Squid). However, you won't have access to other benefits of Squid Premium such as the premium paper backgrounds or future premium features.
Rest assured, if you purchase any of the individual premium features, you will always have access to these features.
To purchase an individual premium feature, simply tap "Want just [premium feature]?" on the premium dialog that appears when accessing the premium feature. For example, you can tap "Import PDF" on the main screen of Squid, then tap "Want just PDF Import?" on the PDF Import premium dialog. This will take you to a screen that explains what PDF Import is and lets you purchase it.