Just like making a website compliant can't hurt with your SEO, making your PDF accessible can't hurt. The Adobe built-in accessibility checker is far from perfect, but at least fixing those areas will get you started.
I probably spend 5 minutes on each 4 or 5, mostly text PDFs we put online. The time goes up evenly depending on the number of pages, and how complex those pages are.
Assuming you have Adobe Acrobat Pro to do your editing:
- Run an Accessibility Full Check. (Quick check is pretty pointless to me)
- Update the meta information in the document properties (keywords, subject, language, etc)
- Make sure tags are added
- Make sure the text is tagged as text, images as images, background stuff as background
- Tag useless fluff (like decoration or design) as background
- Add good alt text to the images
- Make sure in the reading order, the text is ordered properly
- In the content toolbar, make sure the text isn't duplicated or grossly mistranslated
- Use the OCR scanner on scanned pages
For more advanced editing like tables and really oddball Adobe errors, we use a plugin called CommonLook. CommonLook gets the job done, but I hate it almost as much as I hate the Adobe tools.
Get familiar with the Touch Up Reading Order tool, the Tags toolbar, the Reading Order toolbar and the Content toolbar. My job requires fully compliant documents before going out on the web, but anybody could benefit from some simple tagging and document properties.