Creating a plugin that will display text on every page

Use the_content or the_title/single_post_title(?) filter and simply prepend/append what you need. Also take a look at the Action/Filter API Reference.

@Example:

/**
 * Appends the authors initials to the content.
 * Mimics paper magazines that have a trailing short to show the end of the article.
 * Gets appended at the end inside the last paragraph.
 * @param (string) $content
 * @return (string) $content
 */
function wpse28904_append_to_content( $content )
{
    // Only do it for specific templates
    if ( is_page() || is_archive() )
        return $content;

    // Get author initials
    $author="";
    preg_match_all( '/[A-Z]/', get_the_author(), $initials_arr );
    foreach ( $initials_arr[0] as $initials )
        $author .= $initials;
    $author_url     = get_the_author_meta('url');
    if ( $author_url )
    {
        $title  = esc_attr( sprintf(__("Visit %s&#8217;s website"), $author ) );
        $author = "<a href="https://wordpress.stackexchange.com/questions/28904/{$author_url}" title="{$title}" rel="external">{$author}</a>";
    }

    // Append  author initials to end of article
    $content  = preg_replace( "/<p[^>]*><\\/p[^>]*>/", '', $content );
    $position = strrpos( $content, '</p>' );
    $content  = substr_replace( $content, "<sub>{$author}</sub>", $position, -1 );

    return $content;
}
add_filter( 'the_content', 'wpse28904_append_to_content' );

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