Essentially, the information in running headers should be provided in a manner that is accessible by assistive technology so users can more easily orient themselves in a document. However, in "PDF14: Providing running headers and footers in PDF documents", the W3C suggests making running headers "pagination artifacts" using built in "header" features in authoring tools.
This means that while the running header will not be announced by screen readers, any important information included in the running header must be made available elsewhere. For example, a PDF document may have 4 pages. However, if the first page is a title page, then the page numbering might start on the second page, resulting in the running header having a page number of "1". In this scenario, you would need to ensure the actual page number of the file matches the page number in the header or footer.