PDF is safest for final documents
Use PDF for resumes, reports, forms, signed documents, certificates, scans, and submissions where the layout should not change.
DOCX and XLSX are for editing
Use DOCX when someone needs to edit text. Use XLSX when someone needs a spreadsheet workbook. Convert to PDF when the file is final.
Images as documents
Use JPG or PNG when the form asks for an image. Use JPG to PDF or PNG to PDF when the image represents a document, scan, receipt, or form.