A family of Microsoft word processing software products for creating web, email, and print documents.
Word is not a PDF viewer app, this is most likely caused by a file association error, if you download a PDF file and it automatically opens in Word, Word will always convert a PDF to an editable document when it is set as the default for PDF files.
Right click a PDF file and select 'Open With', then choose Edge or your preferred app to open PDF documents, and ensure you select 'Always open PDF files with this app', that will reset the file associations on PDF documents to that app.