Downloading the CSS and re-uploading it as a static resource is unnecessary. You can use the SLDS stylesheets by [adding the `<apex:slds />` tag to your Visualforce page](https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/pages_styling_slds.htm). [This is documented on the SLDS site under Visualforce](https://www.lightningdesignsystem.com/platforms/visualforce/), the download is offered if you want to use the CSS _outside_ of Salesforce.

You can also scope where you want the SLDS styles to be applied.

```html
<apex:page standardController="Account" applyBodyTag="false">
    <apex:slds /> 
 
    <!-- any Visualforce component should be outside SLDS scoping element -->
    <apex:outputField value="{!Account.OwnerId}" /> 
 
    <div class="slds-scope">
    <!-- SLDS markup here -->
    </div>
</apex:page>
 
```

**Additionally,** make sure you're using the appropriate markup for the SLDS class names. Many of the classes that use double hyphens (`--`) have been deprecated.