As per my understanding, you can block the pdf extension type using the UTM block-extension list in the feature-profile settings. There would be no difference between pdf and password protected pdf in terms of extension as such and so it would not be possible as per my understanding.
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Starting with Junos OS Release 18.2R1, the following commands under the [edit security utm feature-profile] hierarchy level are deprecated.
Before you can configure most UTM features, you must first configure the custom objects for the feature in question. Custom objects are global parameters for UTM features. This means that configured custom objects can be applied to all UTM policies where applicable, rather than only to individual policies.
The below document gives you an example configuration of content filtering custom objects:
As already explained, content-filtering works using the extension .pdf which is same for password protected as well as non protected. So there is no way for SRX to differentiate if it is password protected or not using content-filtering.
To keep it short, it is not possible to block specifically password protected pdf files.