Opportunity openOpp = [
    SELECT Id
    FROM Opportunity
    WHERE IsClosed = false
    LIMIT 1
][0];

Quote newQuote = new Quote(
    OpportunityId = openOpp.Id,
    Name = 'doesn't matter'
);

insert newQuote;

// You can't query this object so you have to hard code a quote template ID
// We use a custom setting
Id templateId = '';

String url = '/quote/quoteTemplateDataViewer.apexp?id=' +
    newQuote.Id + '&summlid=' + templateId;

PageReference contentSource = new PageReference(url);
Blob pdf = contentSource.getContentAsPDF();