One potential approach, although not very "dynamic," is to adjust the height to accommodate the three given pages. In this case, I set it to 1970px, roughly based on A4 dimensions at 500px wide = 706px high, totaling about 2118px in height. However, in practice, 1970px seems sufficient for the three assigned pages (you can fine-tune this on a case-by-case basis).
<h1>Hello world</h1>
<div class="box">
<iframe id="iframe1" src="https://ana.mareca.es/wp-content/plugins/algori-pdf-viewer/dist/web/viewer.html?file=https%3A%2F%2Fana.mareca.es%2Fwp-content%2Fuploads%2F2020%2F05%2FPdf-de-prueba-con-varias-paginas.pdf#view=fit"
width="100%" height="1970px"></iframe>
</div>
<h3>I acknowledge that I have reviewed all three pages</h3>