Unfortunately, the correct answer to this problem is don't edit PDFs.
Editing text in a PDF is a last resort. You should only edit PDF text if you don't have the source file and it's a small, one-time change (meaning it is significantly faster to just make the edit instead of tracking down the person or location of the original file).
If you have the original file, always update the source file and re-export the PDF. This way, your source file does not become out-of-date. This also avoids weird things that can happen when trying to directly edit PDF text, like the examples you provided in your question.
If you do not have the original file, stop and ask yourself the following:
- Am I really responsible for making this change, or should I contact the person that made it? (It should be the responsibility of the party that created it. Contact that person and ask them to make the change and re-export)
- Am I expected to maintain and make future changes to this file? (If yes, and you really can't get the source file, strongly consider re-creating it—you'll save time and produce better work in the long run. There are many tools and techniques for doing this, and would be worth a separate question)
- Was this file created by a program? (If yes, consider requesting that the program be updated—never do manual work that a computer can do for you)
In most of these case, you should not edit the PDF, but rather attempt to update the source. Manually editing text in a PDF should only be done when you do not have the original file and you have no other options. It is a last resort.