How to save pdf file from iframe on server?

Question

JavaScript

Good to everyone!
ASP.NET Core MVC + c# web application
The view has the following code:

<form asp-action="AnketSave" enctype="multipart/form-data">
                <h2>Анкета</h2>
                <input type="submit" value="Сохранить" />
                <iframe src="~/Ankets/Anketa.pdf" style="width: 100%; height: 800px;"></iframe>
            </form>

The pdf file has checkboxes that the user can set.
I need to save the modified file via POST to the wwwroot/UserAnket folder.
I can't figure out how to pull it out of the iframe
Thanks in advance!