@tranthanhxhong: Right, that's normal. The exact amount will depend on the size and complexity of your PDFs. (Only up to five tabs are currently kept in memory at a given time.)
If you see a much larger number in Troubleshooting Mode, or a number that continues to increase without ever going down, we'd want to know about that, but what you're showing isn't any indication of a problem.
If you're getting much larger numbers with plugins enabled, you should identify the plugin responsible and report it to the plugin developer (and also let us know).
Upgrade Storage
https://s3.amazonaws.com/zotero.org/images/forums/u9082669/t0l69lqa0sttylcr6u7b.jpg
https://s3.amazonaws.com/zotero.org/images/forums/u9082669/viot47nevdk56i8bq7g2.jpg
If you see a much larger number in Troubleshooting Mode, or a number that continues to increase without ever going down, we'd want to know about that, but what you're showing isn't any indication of a problem.
If you're getting much larger numbers with plugins enabled, you should identify the plugin responsible and report it to the plugin developer (and also let us know).