Every tablix in SSRS has a property that decides if header rows should be repeated on each page of report. You can change this property in Tablix Properties:
However, that property is not displayed correctly when exporting to a format like PDF. If you want to repeat row headings in a tablix exported to file, you must do as follows:
Select the tablix
In the tablix pane, activate Advanced Mode
Every (Static) entry is a single header row displayed in the tablix (e.g., a simple table with one header row have one (Static)). Set the appropriate (Static) entry’s KeepWithGroup property to “After” and the RepeatOnNewPage to “True,” and your PDFs should show header information on each page.