@Gaurav Kheterpal,

Thanks for your repy..

VF Code:

<!--For uploading the document-->
<apex:page controller="Repository_apex">

 <apex:sectionHeader title="Add Documents"/>
 
  <apex:form enctype="multipart/form-data">
    <apex:pageMessages />    
    <apex:actionStatus id="stat" style="align:center;">
          <apex:facet name="start"> 
              <apex:outputPanel styleClass="customPopup" layout="block" >
                   <div ><apex:image style="width:10%;allign:center;" value="{!$Resource.Loading}"/></div>
              </apex:outputPanel>                
          </apex:facet> 
          <apex:facet name="stop"/>
    </apex:actionStatus>
    
    <apex:pageBlock title="Upload a File">
 
      <apex:pageBlockButtons >
        <apex:commandButton action="{!upload}" value="Save"/>
        <apex:commandButton action="{!Documents}" value="Documents"/>
      </apex:pageBlockButtons>
      
      &nbsp;&nbsp;&nbsp;<b>Folder</b>&nbsp;
          <apex:actionRegion >
          <apex:selectList size="1" value="{!FolderName}" id="folder" >
              <apex:actionSupport event="onchange" action="{!GetDocuments}" rerender="Relateddocs" status="stat"/>
              <apex:selectoption itemvalue="Please Select"/>
              <apex:selectoption itemvalue="Legal"/> 
              <apex:selectoption itemvalue="Credit"/>
              <apex:selectoption itemvalue="Operation"/>
              <apex:selectoption itemvalue="HFC"/>
          </apex:selectList>
          </apex:actionRegion>
      
      <apex:pageBlockSection showHeader="false" columns="2" id="block1">    
          <apex:pageBlockSectionItem >              
              <apex:outputText />
          </apex:pageBlockSectionItem>
          <apex:pageBlockSectionItem >
              <apex:outputText />
          </apex:pageBlockSectionItem>
      <apex:pageBlockSectionItem >
          <apex:outputLabel value="Document Name" for="fileName"/>
          <apex:inputText value="{!document.name}" id="fileName"/>
        </apex:pageBlockSectionItem>
 
        <apex:pageBlockSectionItem >
          <apex:outputLabel value="File" for="file"/>
          <apex:inputFile value="{!document.body}" filename="{!document.name}" contentType="{!document.contentType}" id="file" accept="{!document.contentType}"/>
        </apex:pageBlockSectionItem>
 
        <apex:pageBlockSectionItem >
          <apex:outputLabel value="Description" for="description"/>
          <apex:inputTextarea value="{!document.description}" id="description"/>
        </apex:pageBlockSectionItem>
 
        <apex:pageBlockSectionItem >
          <apex:outputLabel value="Keywords" for="keywords"/>
          <apex:inputText value="{!document.keywords}" id="keywords"/>
        </apex:pageBlockSectionItem>
 
      </apex:pageBlockSection>
    </apex:pageBlock>
    
    <apex:pageBlock title="Related Documents">  
      <apex:pageblockTable value="{!RelatedDocuments}" var="RD" id="Relateddocs">
              <apex:column value="{!RD.name}" headerValue="Document Name"/>  
              <apex:column value="{!RD.description}" headerValue="Description"/>            
              <apex:column headerValue="File">                   
                   <apex:outputLink value="https://c.cs5.content.force.com/servlet/servlet.FileDownload?file={!RD.id}" target="_blank">
                    View</apex:outputLink>
               </apex:column>               
          </apex:pageblockTable>
    </apex:pageBlock>
  </apex:form>
</apex:page>

Apex Code:
===============================================================================

//For uploading the document in repository
public class Repository_apex{
 
  public string FolderName{get;set;}
  public list<Document> RelatedDocuments{get;set;}
  //public string Docid{get;set;}
  
  public Document document {
    get {
      if (document == null)
        document = new Document();
      return document;
    }
    set;
  }
  
  //this method will call when user clicks on Save button 
  public PageReference upload() {
    
    if(FolderName != 'Please Select' && document.name!='' && document.Body!=NULL){
        //document.AuthorId = UserInfo.getUserId();
                
        Folder Fold = [Select Id from Folder where Name=:FolderName limit 1];
        
        document.FolderId = Fold.id;
        
        try {
              insert document;
            } catch (DMLException e) {
              ApexPages.addMessage(new ApexPages.message(ApexPages.severity.ERROR,'Error uploading file'));
              return null;
            } finally {
              document.body = null; // clears the viewstate
              document.description = null;
              document.keywords = null;
              document = new Document();
              
              RelatedDocuments = [select id,name,description from Document where Folder.name=:FolderName order by Name];
            }
         
            ApexPages.addMessage(new ApexPages.message(ApexPages.severity.INFO,'File uploaded successfully'));        
        }        
                
      }else
           ApexPages.addMessage(new ApexPages.message(ApexPages.severity.Error,'Folder, Document Name & File fields are mandatory'));
           
       return null;
    }
    
    //this method will call when user clicks on Documents button 
    public PageReference Documents() {
        
        PageReference ProdPage = new PageReference('/015/o');        
        
        return ProdPage;
        
    } 
    
    //this method will call when user changes Folder Value
    public void GetDocuments(){
        RelatedDocuments = [select id,name,description from Document where Folder.name=:FolderName order by Name];
    }    
    
}