Text in a text box disappears upon clicking outside the text box

Well, there's a lot going on here. Part of the confusion is the different terms that are being used, and where you posted. Although you've confirmed you aren't working on a form, you DID post in the Forms forum, so it isn't too surprising you got a lot of advice about forms.

I think I've discovered something by looking at your file and experimenting. You're adding text, probably the only way you've found so far. This is something rather old and creaky called the 'typewriter tool', which not so many use today. However, in Acrobat X it's easy to find under "Add or edit text box". What you add isn't like ordinary text on the page, it's really a kind of comment.

There are lots more kinds of comments under the Comment tab, including some much more powerful tools for adding text. You might find you prefer these, but let's look at the Typewriter problem.

Your problem is that your comments are less than 1% opaque. That is, pretty much transparent, and so they are invisible. How did it happen? Don't know, but I have a guess. Doesn't matter, I may have a fix. The problem is that this opacity thing is a real setting but with no tools for setting it. Now it's set, you have to get rid of it somehow.

1. You say, I think, you have a computer where all is well. On that computer, make a Typewriter text box. Save the PDF, and get it to the problem computer.

2. Open the good PDF on the problem computer.

3. Select the Comment tab. Look for the one you added under the comments list.

4. Double Click on the entry in the comments list and make sure it selects the right one. It must be the typewriter box.

5. Right click on the entry in the comments list and choose Properties.

6. Click on "Make Properties Default" and click OK.

If all is well, the good (100% opacity) setting will now apply to all future typewriter boxes.