Call Visualforce Page from LWC with Parameter

Question

I have a requirement to perform a callout from an LWC to get invoices data and then display as a PDF the data returned. So i have an LWC with a lightning data table and on row action it calls an apex class to do the callout and get the result. In order to do the rendering to PDF, i created a visualforce page which is called from apex. However, i didn't manage to find solution i was getting several errors like "ERROR: Formula Expression is required on the action attributes." or like "common.apex.runtime.impl.executionexception argument cannot be null". Seems like i am doing something wrong when passing the data from Apex to the Visualforce.

so, the issue is:

1. i have got the invoice data of structure:

public class el_invoices{
        @AuraEnabled
        public Boolean Success {get; set;}
        @AuraEnabled
        public String ErrorCode {get; set;}
        @AuraEnabled
        public String ErrorMessage {get; set;}
        @AuraEnabled
        public String BillNo {get; set;}
        @AuraEnabled
        public String BillData {get; set;}
    }

2. Then on specific invoice action i call from my LWC an APEX function showPDF

@AuraEnabled
    public static void showPDF(String s){
        System.debug('in show pdf: '+s);
        navigate(s);
    }
    
    public static blob navigate(String s) {
        PageReference pg = new PageReference('/apex/el_generatePDF');
        pg.getParameters().put('displayText', 'sdfsdf');
        
        pg.setRedirect(true);
        //blob doccontent = pg.getContent();
        //ystem.debug('doccontent '+doccontent);
        system.debug('show pdf finish');
        return pg.getContent();
    }

3. But my visualforce is always blank, like i cannot pass data from my apex class to the "displayText" attribute. Has anyone found solution on such topic? Again, what i want is to make a callout, and show a PDF (not save anything in the database)

<!--<apex:page controller="el_generatePDFController" contentType="application/pdf" renderAs="pdf">-->
<apex:page controller="el_generatePDFController" contentType="application/pdf" renderAs="pdf">

    <apex:form >
        <!-- Page code -->
        <apex:inputhidden id="hidData" value="{!displayText}"/>
        {!displayText}
    </apex:form>
    
</apex:page>

public with sharing class el_generatePDFController {
    
    public String displayText {get; set;}

    public String getdisplayText (){
        /*//System.debug('s: '+ s);
        System.debug('pdfdatastring: '+ pdfdatastring);
        return EncodingUtil.base64Decode(pdfdatastring).toString();
        */
        displayText = 'giorgos';
        return displayText;
    }
}

UDATE: I edit my question to show at a high level the problem that i am facing:

Answer 1

You can't generate a form in the page that would be manipulated into rendering the PDF, because that would be a second round trip. You can either (a) put the content into a page parameter (maximum limit is about 3k characters), or, as I would recommend, use Blob.toPdf() to directly generate the PDF!

Here's a revised version of your method that should help:

public static blob navigate(String s) {
    return Blob.valueOf(s).toPDF();
}

Of course, you have some more work to do, but this should get you started. Just render a full, proper HTML document, pass it to your Apex method, and then you can render the PDF immediately.