Hi - I think you are combining two different things or I've misunderstood. But I'll try:
1. If you want the PDF to not be printed from Sales Quote if certain fields are not having values, use the Report triggers to write your errors and not page triggers. You can use relevant table's OnAfterGetRecord as you'll get all values here straightaway
2. You can alternatively make use of a Subscriber which will be called as soon as the PRINT button is pressed from the page. Once event named "OnBeforeActionEvent" is called via your subscriber, you can do TESTFIELD of various fields here in this snippet only. Only when there are no errors from these TESTFIELDS, will your report request page come up. Sample code screenshot below

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