Use Advanced Search ( Shift+Ctrl+F ) to search across multiple PDFs. For example, search Ultimate Combat AND Core Rulebook for every mention of "critical hit."
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"Gun" (you will get every instance of "longbow," "crossbow," and "gun"). DO SEARCH FOR: "Firearm" or "Misfire" (these are unique to this book). Use Advanced Search ( Shift+Ctrl+F ) to search
Here are the tier-one PDF readers for Pathfinder gaming: pathfinder ultimate combatpdf work