Try this.
# Auth: Christopher Stone
# dCre: 2017/05/10 22:09
# dMod: 2017/05/10 22:12
# Appl: Skim
# Task: Activate "Merge All Windows" to consolidate into tabs of one window.
# Libs: None
# Osax: None
# Tags: @Applescript, @Script, @Skim, @System_Events, @Activate, @Merge_All_Windows, @Consolidate, @Tabs, @One, @Window
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tell application "System Events"
tell application process "Skim"
tell menu bar 1
tell menu bar item "Window"
tell menu 1
tell menu item "Merge All Windows"
perform action "AXPress"
end tell
end tell
end tell
end tell
end tell
end tell
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