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Inside the DOCX templates you can use all variables but ensure that you do not apply any styles to them. The reason is that a Word application can escape certain parts of the variable name with certain tags and the PDF app then will not be able to evaluate such a variable. For instance, $issue.key will be evaluated as a task's key, but if you apply a style to certain part of it ($issue.key) then the PDF application will keep this variable as is and will not substitute its value in the result document.
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3. Special variables ($fieldRenderer.renderFieldById("..."), $fieldRenderer.renderField("<name>” etc.) to use in the template
Use Edit with Notepad++ to change the template instead of MS Word and save like an open XML docx document for these variables to be working.
4. Encoding issue: how can I avoid "??" in the exported documents
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