How to include stylesheet in visualforce page rendered as pdf ?

You're not going to be able to render PDFs properly that were created with Bootstrap. The Flying Saucer Render engine used by Salesforce only supports CSS 2.1 and HTML 4.1.

When you remove render as pdf then bootstrap is available. Also keep in mind that there many css which will not work with renderAs=pdf

When you remove render as pdf then bootstrap is available. Also keep in mind that there many css which will not work with renderAs=pdf so best to use inline CSS.

Thanks

Here is the example:

A Visualforce page rendered as a PDF file displays either in the browser or is downloaded, depending on the browser’s settings. Specific behavior depends on the browser, version, and user settings, and is outside the control of Visualforce.
The following page includes some account details and renders as a PDF file.

<apex:page standardController="Account" renderAs="pdf">

<apex:stylesheet value="{!URLFOR($Resource.Styles,'pdf.css')}"/>

<h1>Welcome to Universal Samples!</h1>

<p>Thank you, <b><apex:outputText value=" {!Account.Name}"/></b>, for
becoming a new account with Universal Samples.</p>

<p>Your account details are:</p>

<table>
<tr><th>Account Name</th>
<td><apex:outputText value="{!Account.Name}"/></td>
</tr>
<tr><th>Account Rep</th>
<td><apex:outputText value="{!Account.Owner.Name}"/></td>
</tr>
<tr><th>Customer Since</th>
<td><apex:outputText value="{0,date,long}">
<apex:param value="{!Account.CreatedDate}"/>
</apex:outputText></td>
</tr>
</table>

</apex:page>

Thanks