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Word files give the fullest check: a PDF can't show tracked changes, hidden text, superscripts or page layout. Old .doc files: in Word, use File → Save As → .docx.
Check a manuscript the way a journal screening desk does before peer review: structure, abbreviations, figure and table citations, ethics and consent statements, declarations, references and citation style (APA, AMA, Vancouver, Harvard, IEEE) and, for double-blind journals, anonymisation. Choose your journal's profile, or adjust the rules to match its guidelines.
Word files give the fullest check: a PDF can't show tracked changes, hidden text, superscripts or page layout. Old .doc files: in Word, use File → Save As → .docx.