If you capture a document through a scanner using Adobe Acrobat Pro 8 (available as a free download from
http://www.techspot.com/downloads/4683-adobe-acrobat-8-free.html - note the serial# mentioned there), the scans naturally occupy the full width of whatever portion of the pages you set the scanner to capture, so centring isn't an issue. And, if you then run Acrobat Pro OCR process on the resulting PDF, it will be fully searchable, the text positioning will exactly match the text in the images and can be selected through them. None of that is possible with Word.
A Word macro can do whatever image alignments you want - including different alignments on odd & even pages. However, it cannot determine the alignment according to any element within the image. So, if your scanned text within the image is misaligned from one image to the next, a macro can't help with that.
FWIW, I've exported PDFs to Word a number of times. Depending on the PDF, each line might come out as a separate physical paragraph. These are easy enough to re-join via Find/Replace. If you replace the paragraph breaks with manual line breaks, it's easier to retain the original's layout & pagination, even though you'll probably be using a different font and/or point size, line spacing, etc. Replacing the paragraph breaks with spaces (or importing PDF conversions that honour intra-paragraph line breaks with spaces) makes for easier copying, etc. later on, but it then also becomes much harder to restore the original page layouts & pagination. Even if you don't import the content via the PDF route, you'll still face all of the same issues.