In order to send documents to a candidate they need to be included in a "Pack" which can be selecting when onboarding a candidate.
To create a new pack template select "Packs" from the "Onboarding menu
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Then click the "New Pack" button
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Give your pack a name then click "Create"
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To add documents to your pack click either of the "Add Documents" buttons
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Don't forget to set your pack type under "Non-Mandatory" fields
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Your pack will then appear on the pack selection screen the next time you add a pack to a candidate or start onboarding