As you noted Acrobat is not a word processing tool. Edit the text if you must then drag the text to the left. It won't be perfectly centered. It should be possible to get the original file to edit, if not it might be better to recreate the file. You can export the pdf to a word file and proceed from there.
You might try to delete the text and create a header that is repeated on each page.
You have some (limited choices) - highlight the text and then right-click and PROPERTIES. From here you can see if you indeed align the text within the frame but this does not always work (depends how the text content was created via the source document). Also, if you wish to create a new text frame then using the touch-up text tool, find a BLANK area inside your PDF (no text, image or anything else) and then hold the ALT key down whilst clicking - new text frame. Don't forget to use the touch-up object tool if you're using Acrobat Pro (not in the Standard version).
Here's one of my documents which might help : http://certitec.com/pdf/acrobat8top10tips.pdf
Thank you for your responses! The header is not the same on each page. And some pages have headers and some don't. The file is 62 pages long. Sigh. I didn't see when I right-clicked for properties a place that allowed me to change the alignment. Am I missing it?
Also, I'm on a Mac. What should I hold down instead of the Alt key? I tried all the obvious keys, but I didn't see anything that allowed me to do a new text frame.
Indeed, choices are limited since Acrobat doesn't see aligned text as such - just blocks. It sounds a hard job to crack. If I have to bring on the pro tools I also use Enfocus Pitstop (www.enfocus.com) to pick-out characteristics of the headers etc (font, size, colour) and then use a command to remove - this application is a plug-in for Acrobat.
Otherwise you could try the CONTENT tool (View/Navigation Panels/Content) and then select HIGHLIGHT CONTENT and drill down the PDF tree - right click to delete content ...
I've on a Mac now with an ALT key (next to command/apple) - my iMac also has an ALT key in the same place?
Take another peek,