According to the PDF specification (v1.7), clause 12.5.2, the Border entry in an annotation dictionary (table 164) specifies the border width in user-space units. However, Adobe Acrobat Reader treats these as device-space units (i.e., pixels).

The attached PDF document defines borders with 1-unit and 5-unit widths; however, in Adobe Acrobat Reader these borders are always 1px or 5px wide, regardless of the zoom level.