How do I build a portfolio of several PDF files using Acrobat XI Standard version?
If you mean a PDF Portfolio, you can't, it's a feature of Pro: http://www.adobe.com/products/acrobat/product-comparison.html
If you mean something else, provide more details and someone should be able to help.
fwiw - the first "Portfolio" feature came out with the Pro and Extended versions of Acrobat 9.
The Pro version of Acrobat X and XI continued to provide the "Portfolio" feature.
The precursor to the "Portfolio" feature was the "Package" feature available in the Pro / 3D version of Acrobat 8.
As stated, the "Package" / "Portfolio" feature was / is exclusive to the Pro version of Acrobat.
It has never been available in the Standard version of Acrobat.