Add all values that are in the database and display on the screen

I do not know if you know how to return the values, but based on my comment up, follow the base code.

$sql = $pdo->prepare("SELECT SUM(valor) FROM anuncios");
$sql->execute();

$ln = $sql->fetchObject();

echo $ln->valor;

The select is this:

SELECT SUM(valor) FROM anuncios

How will you treat and display, there is no way to know with that piece of code. If you want, post full.

<?php require 'pages/header.php'; ?>
<?php
    if(empty($_SESSION['cLogin'])) {
        ?>
        <script type="text/javascript">window.location.href="login.php";</script>   
        <?php
        exit;
    }
?>
<div class="container"> 
    <h1 align="center">CONTROLES</h1>

    <a style="margin-left: 510px;" href="add-anuncio.php" class="btn btn-success">Adicionar Dados</a><br/><br/>

    <table class="table table-striper">
        <thead>
            <tr>
                <th>Foto</th>
                <th>Cliente</th>
                <th>Categoria</tb>
                <th>Valor</th>
                <th>Vencimento</tb>
                <th>Descrição</th>
                <th>Estado</th>
                <th>Ações</th>
            </tr>
        </thead>
        <?php
            require 'classes/anuncios.class.php';
            $a = new Anuncios();
            $anuncios = $a->getMeusAnuncios();

            foreach($anuncios as $anuncio):
        ?>
        <tr>
            <td>
                <?php if(!empty($anuncio['url'])): ?>
                <img src="assets/images/anuncios/<?php echo $anuncio['url']; ?>" width="100" height="40" border="0"/>
                <?php else: ?>
                <img src="assets/images/logo.png" width="100" height="40" border="0"/>
                <?php endif; ?>
            </td>
            <td><?php echo $anuncio['titulo']; ?>
            </td>
            <td>
                Cebola
            </td>
            <td>R$ <?php echo number_format($anuncio['valor'], 2); ?></td>
            <td>
                <?php echo $anuncio['data']; ?>
            </td>
            <td>
                <?php echo $anuncio['descricao']; ?>
            </td>
            <td><?php if($anuncio['estado']==1) {?><strong>PAGOU</strong><?php }else {?><strong>NÃO PAGOU</strong><?php };?></td>
            <td>
                <a href="editar-anuncio.php?id=<?php echo $anuncio['id']; ?>" class="btn btn-warning">Editar</a>
                <a href="excluir-anuncio.php?id=<?php echo $anuncio['id']; ?>" class="btn btn-danger">Excluir</a>
            </td>

        </tr>
        <?php endforeach;
        ?>
        <tr>
        </tr>
            <thead align="center">
                <tr>
                    <th style="text-align:center;">Valor Total:<?php 
                    $sql = $pdo->prepare("SELECT SUM(valor) FROM anuncios WHERE valor = ?");
                    $sql->execute(array($_POST['valor']));

                    $ln = $sql->fetchObject();

                    print_r($ln);

                    ?></th>

                </tr>
        </thead>
</table>

Use AS mysql to create a nickname that is accessible in fetchObject / fetchAssoc :

<tr>
    <th style="text-align:center;">Valor Total:<?php

    $sql = $pdo->prepare("SELECT SUM(valor) AS total FROM anuncios");
    $sql->execute();

    $ln = $sql->fetchObject();

    echo $ln->total;

    ?></th>
</tr>

Use WHERE if you want to filter from a logged-in user, like this:

<th style="text-align:center;">Valor Total:<?php

$sql = $pdo->prepare("SELECT SUM(valor) AS total FROM anuncios WHERE id_usuario=?");

//Passa o ID do usuário para a query
$sql->bindValue(1, $_SESSION['id_do_usuario'], PDO::PARAM_INT);

$sql->execute();

$ln = $sql->fetchObject();

echo $ln->total;

?></th>