Which of the following is a method to reduce the file size of a document?

Compressing pictures in a document is an effective method to reduce its file size. This process involves decreasing the resolution and file size of images within the document, which can significantly decrease the overall size of the file. Images often take up a substantial portion of a document's storage, and by compressing them, you streamline the data necessary to store the document without sacrificing important textual content.

While saving a document as a macro-enabled file type, restricting editing, or hiding markup may affect how the document operates or appears, they do not contribute to reducing the file size in the same tangible way that image compression does. Therefore, focusing on image compression is the most practical approach when looking to optimize document size.