Apple still provides an iWork Update 9.3 that was made available in Dec 2012 that will update the Pages, Numbers, and Keynote applications to their last versions (4.3, 2.3, 5.3). As you installed from the DVD, this update would be compatible with your existing installation and only the applications themselves would update to those newer versions. Direct download of PDF Pages '09 User Guide from Apple. Apple ended sales and support of iWork '09 in early Fall 2013.
Had you not installed your iWork '09 applications from the DVD, but instead, purchased them individually from the Mac App Store during early 2013, there would now be an opportunity using that same Apple ID to use the Mac App Store to get Pages v8.1 (last version for High Sierra) installed. Since there is no Mac App Store record for installing Pages, as far as I know, there is no means to install Pages v8.1 on High Sierra.
The first paragraph may be all you need as Pages '09 v4.3 has far more word processing functionality than Apple implemented in the completely rewritten Pages v8.1. It cannot however, directly open documents created by Pages v5 or later, depending upon the originator to export to a Pages 09 format, or by using Pages for iCloud as a supplement to having a current Pages version on High Sierra.