要固定为自动适应宽度，通过菜单 Edit » Preferences 打开设置窗口，切换 Categories 到 Page Display 标签页。
如图，设置 Default Page Zoom 为 Fit Width，再勾选 Forbid the change of the current Zoom factor during execution of ‘Go to Destination’ actions (these actions can be launched from bookmarks, hyperlinks, java-scripts).
原文：http://www.g2w.me/2011/11/pdf-xchange-viewer-page-zoom-fit-width/
PDF-XChange 介绍：http://xbeta.info/pdf-xchange-viewer.htm