I think your problem solves itself this way:

try {
     $termo = trim($_GET["termo"]);
      if (!empty($termo)) {
$sql = "SELECT * FROM noticias WHERE titulo LIKE :termo OR conteudo LIKE :termo ORDER BY Nid";
$stmt = $DB->prepare($sql);

$likeTermo = "%".$termo."%";
$stmt->bindParam(':termo', $likeTermo, PDO::PARAM_STR);
...

Consider these differences:

  

PDOStatement::bindValue() - > The variable is bound to the reference and will only be evaluated when PDOStatement::execute() is called. And in this case, only values are accepted as arguments.

If you use $stmt->bindValue(":termo", $termo); .
The value must exist, otherwise an error will occur.

  

PDOStatement::bindParam() - > In this case, the expected argument is a reference (variable or constant) and can not be a primitive type such as a string or loose number, function return, or method.

Invalid examples:

$stmt->bindParam(':termo', 10); // Inválido
$stmt->bindParam(':termo', getValue()); // Inválido

Dear friend, I do not know if there are any more errors, but, from this piece of code that you have, I see only one irregularity that can cause this problem, which is the number of parameters linked through bindParam >. In your SQL query you were very specific providing two placeholders , title and : content , but during the part where you linked the values of these placeholders , you passed only one.

Here are some of the fixes I applied to your script:

Example:

<?php

// Valores Extrnos
$key = isset($_GET["keyword"]) ? trim( (string) $_GET["keyword"]) : NULL;

// Conectar PDO
try{

    $opc = array(PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION);
    $pdo = new PDO("mysql:hostname=localhost;dbname=banco_de_dados;", "root", "", $opc);    

    //$sql = "SELECT * FROM noticias WHERE titulo LIKE :titulo OR conteudo LIKE :conteudo ORDER BY id ASC";
    $sql = "SELECT * FROM noticias WHERE titulo LIKE :titulo OR conteudo LIKE :conteudo ORDER BY Nid";
    $stmt = $pdo->prepare($sql);

    $key = "%" . $key . "%";    

    $stmt->bindParam(":titulo", $key, PDO::PARAM_STR);  
    $stmt->bindParam(":conteudo", $key, PDO::PARAM_STR);
    $stmt->execute();

    // Capturar exceção, se haver uma
    } catch(PDOException $e){               
    echo "Erro: " . $e->getMessage();   
}

// Resultados
if(isset($stmt)){
    while($linhas = $stmt->fetch(PDO::FETCH_OBJ)){
        echo  $linhas->titulo . "<br/>";    
        echo  $linhas->conteudo. "<br/>";
    }   
}

?>

NOTE: Working with PDO requires some care, and I also noticed that in this part of your script you have try , but I did not see the part catch if you catch the exception, if any.

References:

PDO :: bindParam - PHP.net

Exceptions - PHP.net

Catch - PHP.net