There used to be at least one plugin that automated the trick to change the attachment's date to the earliest, and thus make it the default for opening, but it did not get updated for Zotero v7 let alone v8.
https://github.com/sharpevo/zotero-pdfkit

There is however a script for the Actions & Tags plugin that does that trick.
https://github.com/crnkv/Zotero-Action-Scripts-Collection/blob/master/Set This as Default PDF.js

The ability to set the default attachment to be opened has been promised.
https://forums.zotero.org/discussion/comment/436279/#Comment_436279

To note only that the above solution did not work fully for me in zotero 8.0.4. Claude gave me a working solution by updating the script, since setting the "added date" of the pdf file to be the earliest date was not enough. Here is the code I added to the Actions & Tags plugin.

/**
* @file Set This as Default PDF
* @version 0.4 (Zotero 8 — URL-aware fix)
* @usage Select one single PDF attachment, then trigger from the context menu.
* Menu Label suggestion: Set This as Default PDF
*
* HOW ZOTERO PICKS THE DEFAULT PDF (priority order):
* 1. Oldest PDF whose URL matches the parent item's URL
* 2. Oldest PDF with any other URL
* 3. Oldest non-PDF matching parent URL
* 4. Oldest non-PDF with other URL
*
* This script wins on BOTH axes: it gives the target PDF the parent's URL
* (priority #1), removes that URL from siblings (so they fall to priority #2),
* AND backdates the target so it wins on age too.
*/

const SCRIPTNAME = "Set Default PDF";

function popup(msg, timeoutSecs = 4) {
const pw = new Zotero.ProgressWindow({ closeOnClick: true });
pw.changeHeadline(SCRIPTNAME);
pw.addDescription(msg);
pw.show();
pw.startCloseTimer(timeoutSecs * 1000);
}

function showAlert(msg) {
Zotero.alert(null, SCRIPTNAME, msg);
}

function getEarliestDate(attachments) {
return attachments.reduce((earliest, att) => {
const d = new Date(att.dateAdded + "Z");
return d < earliest ? d : earliest;
}, new Date());
}

async function setAsDefault(targetItem) {
const parent = targetItem.parentItem;
if (!parent) {
showAlert("This attachment has no parent item.");
return;
}

// Collect all PDF siblings
const allAttachments = Zotero.Items.get(parent.getAttachments());
const pdfAttachments = allAttachments.filter(
a => a.attachmentContentType === "application/pdf"
);

if (pdfAttachments.length < 2) {
showAlert("Only one PDF is attached — nothing to change.");
return;
}

// Get the parent's URL (this is what Zotero matches against)
const parentURL = parent.getField("url") || "";

// Step 1: Give the target PDF the parent's URL so it wins priority #1
targetItem.setField("url", parentURL);

// Step 2: Clear the parent URL from all other PDFs so they can't compete
for (const att of pdfAttachments) {
if (att.id === targetItem.id) continue;
const attURL = att.getField("url") || "";
if (attURL === parentURL && parentURL !== "") {
att.setField("url", "");
await att.saveTx({ skipDateModifiedUpdate: true });
}
}

// Step 3: Backdate the target to also win on age (belt AND suspenders)
const earliestDate = getEarliestDate(pdfAttachments);
earliestDate.setSeconds(earliestDate.getSeconds() - 1);
const newDateString = earliestDate
.toISOString()
.slice(0, 19)
.replace("T", " ");
targetItem.setField("dateAdded", newDateString);

await targetItem.saveTx({ skipDateModifiedUpdate: true });

popup(`✓ "${targetItem.getField("title") || "This PDF"}" is now the default.`);
}

// ─── Main ─────────────────────────────────────────────────────────────────────
(async () => {
const theItem = (typeof item !== "undefined") ? item : null;
if (!theItem) return; // "all items" pass — skip

const isPDF =
typeof theItem.isPDFAttachment === "function"
? theItem.isPDFAttachment()
: theItem.attachmentContentType === "application/pdf";

if (!isPDF) {
showAlert("Please select a PDF attachment, not a regular library item.");
return;
}

await setAsDefault(theItem);
})();