Overlapping nodes in a tree using TikZ

You can make the sibling distance depend on the level, see the second example on p. 330 of the pgfmanual v3.1.4. For instance, you can add

level/.style={sibling distance={8cm/max(2,#1)}}

to the options of the tikzpicture:

\documentclass[
    12pt,           
    oneside,                
    a4paper,            
    english,        
    french,     
    spanish,            
    brazil              
    ]{abntex2}
\usepackage{float}
\usepackage{tikz}
\usetikzlibrary{calc, shapes,positioning}
\usepackage[lmargin=2.5cm,rmargin=2.5cm,tmargin=2.5cm,bmargin=2.5cm]{geometry}

\begin{document}

\begin{figure}[H]
\centering
\tikzset{thick,
         tree node/.style = {align=center, inner sep=0pt, font = \footnotesize},
every label/.append style = {font=\footnotesize},
                 PL/.style = {draw, circle, minimum size = 10mm, inner sep=0pt,
                             top color=white, bottom color=blue!20,align=center},
               ENL/.style = {% edge node left
                             font=\footnotesize, left=1pt},
               ENR/.style = {% edge node right
                             font=\footnotesize, right=1pt},
                     grow = down,
         level distance = 2cm
           }
\fbox{
\begin{tikzpicture}[every node/.style={font=\scriptsize},
level/.style={sibling distance={8cm/max(2,#1)}}]
\node [PL] {$1$}
    child{node [PL] {$2$}
    child{node [PL] {$3$}
                 child{node [PL] {$4$}}
          child{node [PL] {$5$}}}
            child{node [PL] {$6$}
                 child{node [PL] {$7$}}
          child{node [PL] {$8$}}}
    }
    child{node [PL,] {$9$}
   };
\end{tikzpicture}
}
\caption{\'Arvore de busca do exemplo num\'erico.}
\label{fig:tree}
\end{figure}

\end{document}

Using the max function makes sure that the distance only decreases once the level is greater than 2.

You can avoid to your problem, if you draw your tree with the forest package:

\documentclass[12pt,oneside,a4paper,
               english,french,spanish,brazil]{abntex2}
\usepackage[margin=2.5cm]{geometry}
\usepackage{forest}
\usetikzlibrary{backgrounds}

\begin{document}
    \begin{figure}[ht]
    \centering
\begin{forest}
for tree = {
% node style
    draw, circle,
    minimum size = 10mm,
    inner sep=0pt,
    top color=white, bottom color=blue!20,
    font=\scriptsize,
    math content,
% tree style
    s sep=6mm,
    l sep=12mm,
% frame around tree
tikz+={\tikzset{background rectangle/.style={draw, ultra thin},
                show background rectangle}}
             }
% tree
[1
    [2
        [3
            [4]
            [5]
        ]
        [6
            [7]
            [8]
        ]
    ]
    [9,fit=band]
]
\end{forest}
\caption{\'Arvore de busca do exemplo num\'erico.}
\label{fig:tree}
    \end{figure}
\end{document}

Forest is a pgf/TikZ-based package for drawing linguistic (and other kinds of) trees.