adding borders in pages document

Pages View menu : Show Layout. Insert a Text box, set its Arrange panel Text Wrap to None. Resize the Text box to fit flush with the document layout guides for the body text.

On the Style tab, click the Border : No Border selector and choose Border. Then set the Line style, color, and thickness to suit. Click and backspace on the Text box's place holder text "Type to enter text" so it is no longer present.

With the Text box still selected, visit the Arrange menu : Section Layouts : Move Object to Section Layout. This places your document border behind your body text. The document border will appear on every page of that section, and if you want it to appear on other document sections, you will need to visit the first page of the new section, click the Documents toolbar item : Section : ☑︎ Match previous section.

View menu : Hide Layout and you are done.