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Jayesh Deo 2 
The complete code is not included in the question. So based on my understanding, looks like you are trying to use the add() method on a List<Messaging.EmailFileAttachment> object, which does not exist. To add a PDF attachment to an email, you need to use the add() method on an instance of Messaging.EmailFileAttachment.
This related example can help you:
Messaging.SingleEmailMessage email = new Messaging.SingleEmailMessage(); // Set the recipient, subject, and body of the email email.setToAddresses(new List<String>{ 'example@salesforce.com' }); email.setSubject('Sample Email with Attachment'); email.setPlainTextBody('Please see the attached PDF.'); // Create an instance of EmailFileAttachment Messaging.EmailFileAttachment attachment = new Messaging.EmailFileAttachment(); attachment.setFileName('Attachment.pdf'); attachment.setBody(blob.valueOf('PDF content')); // Add the attachment to the email email.setFileAttachments(new List<Messaging.EmailFileAttachment>{ attachment }); // Send the email Messaging.SendEmailResult[] results = Messaging.sendEmail(new List<Messaging.SingleEmailMessage>{ email });If this information helps, please mark the answer as best. Thank you
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The complete code is not included in the question. So based on my understanding, looks like you are trying to use the add() method on a List<Messaging.EmailFileAttachment> object, which does not exist. To add a PDF attachment to an email, you need to use the add() method on an instance of Messaging.EmailFileAttachment.
This related example can help you:
Messaging.SingleEmailMessage email = new Messaging.SingleEmailMessage(); // Set the recipient, subject, and body of the email email.setToAddresses(new List<String>{ 'example@salesforce.com' }); email.setSubject('Sample Email with Attachment'); email.setPlainTextBody('Please see the attached PDF.'); // Create an instance of EmailFileAttachment Messaging.EmailFileAttachment attachment = new Messaging.EmailFileAttachment(); attachment.setFileName('Attachment.pdf'); attachment.setBody(blob.valueOf('PDF content')); // Add the attachment to the email email.setFileAttachments(new List<Messaging.EmailFileAttachment>{ attachment }); // Send the email Messaging.SendEmailResult[] results = Messaging.sendEmail(new List<Messaging.SingleEmailMessage>{ email });If this information helps, please mark the answer as best. Thank you
Thanks for your Answer! It helped.
I updated below highlighted declared object and after that code ran without error.