If this is the case with you, you might be interested in taking a look at MuPDF, which is a simple, lightweight PDF reader. It hardly uses any resources at all and it shows the documents quite well.
MuPDF runs without any bars or controls that might make it difficult to read your document. In order to turn the page, rotate the document or carry out any other function, you have to use the keyboard shortcuts that appear in the About section in the menu.
In addition to the tools mentioned above, which allow you to turn the page and rotate the document, MuPDF allows you to zoom, jump forward or backwards ten pages, go to specific page and add bookmarks.