PDO num_rows count

I have this select input that gives me the option to choose a name of all the ones that can be removed:

<td width="50%">
    <select class="form-control" id="acro_proyecto" name="acro_proyecto" required>

    <option value="<?php echo isset($obj_categoria) ? $obj_categoria->__GET('proyecto') : ''; ?>"><?php echo isset($obj_categoria) ? $obj_categoria->__GET('proyecto') : ''; ?> </option>
            <?php
            $pdo = new PDO('mysql:host=localhost;dbname=dmsgeneratorcode', 'root', '');
            $pdo->setAttribute(PDO::MYSQL_ATTR_INIT_COMMAND, "SET NAMES 'utf8'");
            $stmt = $pdo->prepare('Select acro_proyecto from proyectos');
            $stmt->execute();
            while ($row = $stmt->fetch(PDO::FETCH_ASSOC)) {
                echo '<option>' . $row['acro_proyecto'] . '</option>';
            }
            ?>
    </select>
</td>

Now, once you have been chosen, you have to automatically paint the maximum number of times that is automatically entered in the following input:

<td width="50%">
    <input class="form-control numerodocumento" type="text" name="num_documento" id="num_documento" value="
        <?php
        $pdo = new PDO('mysql:host=localhost;dbname=dmsgeneratorcode', 'root', '');
        $pdo->setAttribute(PDO::MYSQL_ATTR_INIT_COMMAND, "SET NAMES 'utf8'");
        $stmt = $pdo->prepare('Select MAX(num_documento) from documento where proyecto='echo isset($obj_categoria) ? $obj_categoria->__GET('proyecto') : '';);
        $stmt->execute();
        while ($row = $stmt->fetch(PDO::FETCH_ASSOC)) {
            echo '<option>' . $row['num_documento'] . '</option>';
        }
        ?>" readonly />
</td>

But the line where it collects the value puts it in red like the syntax is wrong and that's obvious. However, I do not know how to put it right so that it collects the data from the field previously introduced. I get this error in the input in the web application:

Parse error: syntax error, unexpected 'echo' (T_ECHO), expecting ',' or ')'