You will need to add a Unicode keyboard. System Preferences > Keyboard > Input Sources. Click the + button in the lower left. Scroll all the way to the end and select Others > Unicode Hex Input. Select and Add this. Check the box that says Show input menu in menu bar. Exit System Preferences.
From the Finder menu, select your recent Unicode Hex Input source (it is a black box with white U+ in it). Now, position your cursor in your table cell between the characters where you want to insert a tab character. Press and hold your option key, then type 0009 (zeroes). This is the Unicode sequence for the tab control character. Space will open in your text. Tabs in table cells survive PDF export. The tab mark will not appear with Show invisibles enabled.
Switch back to your normal country input source in that Finder input source menu.