Remove or Fix Broken Links

I started with the simple items: I removed a link to an old advertisement and text containing links to PDF files.
The task unexpectedly required more work. In the end, I fixed 23 links rather than 15. The problem was caused by an unfinished customer registration feature, which had to be removed.
There were also blog links throughout the site that produced a broken image link. Finding and replacing them took considerable time, but the site now has no broken links.
As a bonus, I also fixed links that pointed through redirects.

The site had approximately 75 pages without search-friendly URLs. These pages were effectively dead weight: they were not indexed and provided no benefit.

I eventually found the cause. Values containing an invisible space had been copied into the database. That space prevented search-friendly URLs from being generated and caused all of the related problems.

I also noticed automatically generated <p "=""> tags. Because they contained three quotation marks, they could seriously interfere with how the site's content was parsed.

I removed all of the invisible spaces from the database values.
I also replaced <p "=""> with a regular <p> tag.

RESULT:
All of the pages that had previously been blocked or shown as broken became accessible on the site.