Capture data with Javascript in a modal

Friends like that, I need your help, recently I venture into php.

I would like to capture data in a modal.

I generate a table with data from the database with some buttons, by clicking on those buttons I open a modal. I would like to capture the data obtained from the database and capture it in a txt. How could I do?

This is the table where I generate data

<div class="box">
        <div class="box-header with-border">
          <h3 class="box-title">Servicios de Unidad</h3>

          <button type="submit" class="btn btn-success btn-xs pull-right" value="Añadir" data-toggle="modal" data-target="#modal-default"><i class="fa fa-plus"></i> Añadir</button>

        </div>
        <!-- /.box-header -->
        <div class="box-body">
          <table class="table table-bordered">
            <tr>
              <th style="width: 10px">#</th>
              <th>Nombre Unidad</th>
              <th>Objetivo</th>
              <th>Acción</th>
            </tr>
            <?php

            $con = ConexionBD();
            $query= "SELECT * FROM unidades";
            $ejecutar = mysqli_query($con,$query);




            while($mostrar = mysqli_fetch_array($ejecutar))
            { 
             ?>




            <tr>
              <td><?php echo $mostrar['idunidades']; ?> </td>
              <td><?php echo $mostrar['nombreu']; ?> </td>
              <td><?php echo $mostrar['objetivou']; ?> </td>
              <td width="300 px" align="Center">
                <button type="button" class="btn btn-info btn-sm" id="editar" value="Asignar" data-toggle="modal" data-target="#modal-agregar" data-id=17><i class="fa fa-circle-o"></i>  Asignar</button> <button type="button" class="btn btn-warning btn-sm" value="Editar" data-toggle="modal" data-target="#modal-default"><i class="fa fa-edit"></i>  Editar  </button>  <button type="button" class="btn btn-danger btn-sm" value="Eliminar" data-toggle="modal" data-target="#modal-eliminar"><i class="fa fa-remove"></i> Eliminar</button></td>
            </tr>

              <?php
                }
              ?>
          </table>
        </div>

This is the modal

<div class="modal fade" id="modal-agregar">
      <div class="modal-dialog modal-bg">
        <div class="modal-content">
          <div class="modal-header">
            <button type="button" class="close" data-dismiss="modal" aria-label="Close">
              <span aria-hidden="true">&times;</span></button>
            <h4 class="modal-title">Detalle Unidad</h4>
          </div>
          <div class="modal-body">
            <div class="box box-primary">

<form action="../Modelo/ModeloDetalleUnidad.php" method="POST" id="frmaval" name="frmaval" enctype="multipart/form-data" autocomplete="off">        
  <!-- Small boxes (Stat box) -->
  <div class="box-body">

            <div class="form-group">
              <label for="lbloferta">Oferta: </label>
               <input type="text" class="form-control" id="txtoferta" name="txtoferta" placeholder="Descripcion de la Red Social...">
            </div>

            <div class="form-group">
              <label for="lbldescripciono">Descripción: </label>
               <input type="text" class="form-control" id="txtdescripciono" name="txtdescripciono" placeholder="Descripcion de la Red Social...">
            </div>

            <div class="form-group">
              <label for="lblfoto">Imagen: </label>
              <input type="file" class="form-control" id="imagen" name="imagen" placeholder="Foto" accept="image/x-png,image/gif,image/jpeg">
            </div>
           <!-- <div class="form-group">
              <label for="lblfoto">Logo de la Unidad:</label>
              <input type="file" class="form-control" id="logou" name="logou" placeholder="Logo" accept="image/x-png,image/gif,image/jpeg">
            </div>
            <div class="checkbox">
              <label>
                <input type="checkbox"> Check me out
              </label>
            </div>-->



          </div>
          <!-- /.box-body -->

          <div class="box-footer">

            <input type="submit" name="Guardar" id="Guardar" value="Guardar" class="btn btn-primary">
          </div>

          </form>
      </div>
          </div>

        </div>
        <!-- /.modal-content -->
      </div>
      <!-- /.modal-dialog -->
    </div>