• https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/pages_email_custom_controller.htm
• https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/pages_email_sending_attachments.htm

<apex:page controller="sendEmail">
    <apex:messages />
    <apex:pageBlock title="Send an Email to Your {!account.name} Representatives">
        <p>Fill out the fields below to test how you might send an email to a user.</p>
 
        <apex:dataTable value="{!account.Contacts}" var="contact" border="1">
            <apex:column >
                <apex:facet name="header">Name</apex:facet>
                {!contact.Name}</apex:column>
            <apex:column >
                <apex:facet name="header">Email</apex:facet>
                {!contact.Email}</apex:column>
        </apex:dataTable>
   
        <apex:form ><br/><br/>
            <apex:outputLabel value="Subject" for="Subject"/>: <br/>    
            <apex:inputText value="{!subject}" id="Subject" maxlength="80"/>
            <br/><br/>
 
            <apex:outputLabel value="Body" for="Body"/>: <br/>    
            <apex:inputTextarea value="{!body}" id="Body" rows="10" cols="80"/>          
            <br/><br/>
 
            <apex:commandButton value="Send Email" action="{!send}"/>
        </apex:form>
    </apex:pageBlock>
 
    <apex:pageBlock title="Preview the Attachment for {!account.name}">
        <c:attachment />
    </apex:pageBlock></apex:page>

public class sendEmail {
    public String subject {get; set;}
    public String body { get; set; }
 
    private final Account account;
 
    // Create a constructor that populates the Account object
   
    public sendEmail() {
        account = [SELECT Name,
                  (SELECT Contact.Name, Contact.Email FROM Account.Contacts)
                   FROM Account
                   WHERE Id = :ApexPages.currentPage().getParameters().get('id')];
    }
 
    public Account getAccount() {
        return account;
    }
 
    public PageReference send() {
        // Define the email
   
        Messaging.SingleEmailMessage email = new Messaging.SingleEmailMessage();
       
 
        // Reference the attachment page and pass in the account ID
       
        PageReference pdf =  Page.EmailAttachment;
         Blob b = pdf.getContent();
        System.debug('____________________________Pdf___________________________'+pdf);
        pdf.getParameters().put('id',(String)account.id);
        pdf.setRedirect(true);
 
     
        System.debug('____________________________Blob___________________________'+b);
 
        // Create the email attachment
        Messaging.EmailFileAttachment efa = new Messaging.EmailFileAttachment();
        efa.setFileName('attachment.pdf');
        efa.setBody(b);
 
        String addresses;
        if (account.Contacts[0].Email != null) {
            addresses = account.Contacts[0].Email;
            // Loop through the whole list of contacts and their emails
   
            for (Integer i = 1; i < account.Contacts.size(); i++) {
                if (account.Contacts[i].Email != null) {
                    addresses += ':' + account.Contacts[i].Email;
                }
            }
        }
 
        String[] toAddresses = addresses.split(':', 0);
 
        // Sets the paramaters of the email
   
        email.setSubject( subject );
        email.setToAddresses( toAddresses );
        email.setPlainTextBody( body );
 
        email.setFileAttachments(new Messaging.EmailFileAttachment[] {efa});// Sends the email
   
        Messaging.SendEmailResult [] r =
            Messaging.sendEmail(new Messaging.SingleEmailMessage[] {email});  
       
        return null;
    }
}