Last week there was an update to Google Play Books that finally allowed users the ability to upload PDFs. However, that appears to be a short-lived feature as it’s gone in this week’s update. According to a spokesperson at Google, the reason that it was removed is that it was considered experimental and it should come back in a later update.
The 3.1.23 update offers some lower light reading improvements as well as bug and performance fixes. If uploading PDFs is important to you, you can always choose to ignore the update. Other than that you can still use the web interface for your uploading convenience.
source: Android Police