How to paste from clipboard into a Preview edited PDF file
Can you confirm that the text you are viewing is actual text, rather than an image? For example, I opened a PDF in Preview, drag-selected text, right-clicked to choose Copy. I was then able to paste that text into a text editor.

Now, with respect to editing, click the Show Markup Toolbar at the top-right. From here, you can create a Text object, paste your clipboard text, and then adjust the text size and color.

With respect to limited editing, keep in mind that a PDF is generally a compiled format (i.e., the layout for that PDF was created in another application). This is easy to forget when PDF viewers like Acrobat and Preview provide seemingly easy tools for editing and applying annotations. (I suppose that you could also try editing with Illustrator, though that could potentially get quite messy, depending upon the source of the PDF).

As a rule, it's better to edit a given document in its original application, and then recompile into PDF. However, if you have frequent need to perform touchups to PDF, then the Adobe Acrobat application is recommended.