MalformedPackageError

The file is not a package this library can safely read.

Raised for a damaged or inconsistent ZIP envelope, an encrypted or unsupported member, a member name that cannot denote one part, and a [Content_Types].xml or relationship graph that does not describe a readable package. Validation runs before any XML is parsed or any output is replaced, so this is a safe refusal: nothing was read into the object model and nothing on disk was touched.

It does not mean the file is too large. Size and member-count ceilings were removed once Word was measured opening documents past all of them.