Someone sent you a .doc and you do not have Word, which is a licence you were not planning to buy to read one letter. Upload it to the Word viewer and it opens in this tab: DOC, DOCX, ODT, RTF and the template variants, up to 15 MB. The file is deleted after you close it.

Then there is the part nobody warns you about. What you see will not always match what the author saw, and the reasons are worth knowing before you conclude the file is damaged.

Why the page breaks land somewhere else

A .docx is a ZIP archive of XML parts. Rename one to .zip, open it, and you find the text, the styles and the images sitting there as separate files. What you almost never find is the fonts. Word records the font name, not the font itself, so a document asking for a typeface the rendering machine does not have is measured against a substitute. Character widths change, lines rewrap, and four pages become five. Nothing broke. The pagination was never stored in the file to begin with.

RTF sits at the other extreme. It is plain text markup you can read in Notepad, and it stores pictures as hexadecimal digits with no compression at all. That is why a document with three photographs can be several times larger as RTF than as DOCX, and why an RTF that seems far too big for its content usually is not corrupt.

Fields show whatever was there last time

Page numbers, cross-references, dates and a table of contents are not text. They are fields, instructions Word re-evaluates when you refresh them or print. In between, the document stores the last result. Open one whose author inserted a date field in 2019 and never touched it again and 2019 is what appears. A contents page pointing at page 12 is a statement about the author's pagination, not about yours.

What is still in the document after it was edited

Deleting a paragraph with track changes switched on does not remove the paragraph, it marks it. Comments, earlier revisions and author names travel with the file, and any viewer that renders the markup will put them on screen. If you are about to forward a contract that arrived by email, it is better to find that out now than after sending.

When it will not open

It asks for a password to open. That document is encrypted and nothing renders it without the password. "Restrict editing" is a different mechanism entirely: a flag, not encryption, and the text displays normally.
It is a .docm. Macro-enabled documents are accepted and the macros do not run. A viewer draws the document, it does not execute it.
The pictures were linked instead of embedded. Word saved a path to a file on someone else's disk. The frame renders because the frame is in the document; the photograph is not.
It is a .doc from Word 95 or earlier. The binary format changed at Word 97. Anything older is a different format wearing a familiar extension.

If Word files are a recurring problem for you rather than a one-off, the Word viewer is the page worth keeping.