Is it possible to edit a PDF file directly?

You can use (Open Source) qpdf utility (available for Linux, Windows and MacOS X) to unpack the PDF into a more readable format. From there you can go and try some of the other advices from the other answers:

qpdf.exe ^
   --qdf ^
     input.pdf ^
     output.pdf

The file oUtput.pdf will have uncompressed object streams, all objects re-numbered and re-sorted in an ascending order, and some helpful comments sprinkled into the file. The file can be edited in a text editor (if it doesn't mess with the remaining binary sections).

"It Depends."

You'll probably need a couple things: First of all, the text can't have been rasterized. If that's the case, then all bets are off. Second, the entire font must have been embedded. If the font was subsetted (which is most often the case) then you may not have the required glyphs. Finally, you'd probably want to limit the size of the textarea being modified to be as small as possible, just to prevent having to deal with large amounts of reflow. You'd want as much whitespace around the plcaeholder as possible.

Now, this probably won't be something that you'll be able to do with a simple text editor, but there might be some PDF maniulating tools that can do the substiution for you.

If you are willing to get your hands dirty; iText should work.

There are examples which cover a wide range of topics and should get you pointed in the right direction.

Note the example below; using the document.add method to add a Paragraph into an existing PDF document.

protected void createPdf(String filename)
        throws IOException, DocumentException, SQLException {
        // Open the database connection
        DatabaseConnection connection = new HsqldbConnection("filmfestival");
        // step 1
        Document document = new Document();
        // step 2
        PdfWriter.getInstance(document, new FileOutputStream(filename));
        // step 3
        document.open();
        // step 4
        // Add text with a local destination
        Paragraph p = new Paragraph();
        Chunk top = new Chunk("Country List", FilmFonts.BOLD);
        top.setLocalDestination("top");
        p.add(top);
        document.add(p);
        // Add text with a link to an external URL
        Chunk imdb = new Chunk("Internet Movie Database", FilmFonts.ITALIC);
        imdb.setAction(new PdfAction(new URL("http://www.imdb.com/")));
        p = new Paragraph(
            "Click on a country, and you'll get a list of movies, containing links to the ");
        p.add(imdb);
        p.add(".");
        document.add(p);
        // Add text with a remote goto
        p = new Paragraph("This list can be found in a ");
        Chunk page1 = new Chunk("separate document");
        page1.setAction(new PdfAction("movie_links_1.pdf", 1));
        p.add(page1);
        p.add(".");
        document.add(p);
        document.add(Chunk.NEWLINE);
        // Get a list with countries from the database
        Statement stm = connection.createStatement();
        ResultSet rs = stm.executeQuery(
            "SELECT DISTINCT mc.country_id, c.country, count(*) AS c "
            + "FROM film_country c, film_movie_country mc WHERE c.id = mc.country_id "
            + "GROUP BY mc.country_id, country ORDER BY c DESC");
        // Loop over the countries
        while (rs.next()) {
            Paragraph country = new Paragraph(rs.getString("country"));
            country.add(": ");
            Chunk link = new Chunk(String.format("%d movies", rs.getInt("c")));
            link.setAction(
                PdfAction.gotoRemotePage("movie_links_1.pdf", rs.getString("country_id"), false, true));
            country.add(link);
            document.add(country);
        }
        document.add(Chunk.NEWLINE);
        // Add text with a local goto
        p = new Paragraph("Go to ");
        top = new Chunk("top");
        top.setAction(PdfAction.gotoLocalPage("top", false));
        p.add(top);
        p.add(".");
        document.add(p);
        // step 5
        document.close();
        // Close the database connection
        connection.close();
    }