Form fields aren't visible

"when a form is filled in using Mac Preview, then the data isn't visible until you click on the field. The workaround for this is:"

Use this script in the JavaScript Console or in an Action, it works fine:

/*
Folder-level JavaScript file: Preview_fix.js
Purpose: Correct field appearances corrupted by the Preview application on the Mac
Author: George Johnson, acroscript@gmail.com, inspired by a similar script by Joel Geraci
Version: 1.0.0
Date last edited: 2010/12/18
Compatible with Acrobat/Reader 6-10
*/

// Create namespace to prevent potential name conflicts
var COM_ACROSCRIPT = {};

// This function does the work
COM_ACROSCRIPT.correctFieldAppearances = function (doc) {

var i, j,
f_name, f, fp, nWidgets, fw, cc,
c1 = color.blue, c2 = color.yellow; // These can be any color, as long as they are different

// Suspend redrawing field appearances until the end of this script
doc.delay = true;

// Loop through all of the fields in the document
for (i = 0; i < doc.numFields; i += 1) {

// Get the name of the current field
f_name = doc.getNthFieldName(i);

// Get a reference to the current field
f = doc.getField(f_name);

switch (f.type) {

case "button" : break; // No need to process buttons
case "signature" : break; // Digital signatures will not be present if saved with Preview

default : // text, checkbox, radiobutton, combobox, listbox

// Get the page numbers on which the current field's widgets reside
// typeof = number (for single widget) or object (array, more than one widget with the same name)
fp = doc.getField(f_name).page;

// Determine the number of widgets for the current field
nWidgets = (typeof fp === "number") ? 1 : fp.length;

// Loop through the current field's widgets
for (j = 0; j < nWidgets; j += 1) {

// Get a reference to the current widget
fw = doc.getField(f_name + "." + j);

// Save the current widget's border color
cc = fw.strokeColor;

// Set the current widget's stroke color to a temporary color,
// either c1 or c2. Choose c2 if the current color equals c1
fw.strokeColor = color.equal(cc, c1) ? c2 : c1;

// Set the widget stroke color back to the original border color
fw.strokeColor = cc;
}

break;
}
}

// Allow Acrobat/Reader to redraw the field appearances
doc.delay = false;

app.alert("All fields have been processed.", 3, 0);
} // That's all, folks!

COM_ACROSCRIPT.correctFieldAppearances(this);

"Sadly we can't control which program a client uses when they fill a form in"

Of course you can, see this tuto (copy-paste this URL in Google Translate if you can't read French):

https://www.abracadabrapdf.net/ressources-et-tutos/js-et-formulaires-ressources/forcer-utilisation-pdf-avec-acrobat-reader/

How (and where) were the fields filled in?

Can you see the data when you click in the fields?