Display a Blob as a PDF in a Salesforce Visualforce Page Using Base64 Encoding

How the Solution Works

The Visualforce page uses an <iframe> element with its src attribute set to a data: URI. The URI combines the ContentType from the Attachment record (for example, application/pdf) and the Base64-encoded body string provided by the Apex controller.
The Apex controller queries the Attachment record by ID and exposes two properties: att (the Attachment record including Body and ContentType) and pdf (the Base64-encoded string of the attachment body using EncodingUtil.Base64Encode()).

Visualforce Page

<apex:page controller="ViewPdf">
    <iframe src="data:{!att.ContentType};base64,{!pdf}" ></iframe>
</apex:page>

Note: The data: URI may not be supported in all browsers. Internet Explorer does not support data: URIs for <iframe>. Chrome has a 2MB limit. Browsers enforce their own size limits, which Salesforce cannot control.

Apex Controller

The controller lazily loads the Attachment record on first access and exposes the Base64-encoded body:

public class ViewPdf {
    public Attachment att {
        get {
            if (att == null) {
                att = [SELECT Body, ContentType, Name FROM Attachment WHERE ID = '00PG0000004COZU'];
            }
            return att;
        }
        private set;
    }
    public String pdf {
        get {
            return EncodingUtil.Base64Encode(att.body);
        }
    }
}

Replace the Attachment ID 00PG0000004COZU with the ID of your Attachment record.

Workaround for the Chrome 2MB data: URI Limit

To avoid the Chrome 2MB limit, create an Object URL from the Blob instead of using a data: URI directly. This approach uses the window.URL.createObjectURL() browser API to generate a temporary URL for the Blob. This requires JavaScript in your Visualforce page to decode the Base64 string, create a Uint8Array, generate a Blob, and set the Object URL as the iframe src.