If you point your mouse over the file description on the left, the arrow next to it will turn yellow. Click on the file description or the yellow arrow - and the file will open in your browser. For items on the EU-, US-, or Canadian server only a disclaimer page will initially open where you have to agree that you are legally entitled to download.
Better: right-click (PC) or option-click (Mac) and save the file on your computer. This allows you to choose a program to open the file and may circumvent many problems in the browser window. Try to open the file with a recent Adobe Reader (free software: http://get.adobe.com/reader/). There are compatibility issues with some newer pdf-files and Mac-Preview, and Acrobat portfolios are also not recognized.
Incomplete files (especially in MS explorer): If a file is repeatedly truncated, clear the browser cache.
Files on imslp are collected from many different sources. You have to take account of this to obtain the best results.
Most foolproof way to print: Make sure, in the Adobe Reader Print menu "fit to printable area" has been chosen. This will avoid cut-off from the margins but on some scans with already large margins, the margins will now be even larger and the printed music even smaller.
To increase the printed area on such files, you can select "none" instead of "fit to printable area" and also select "Auto-rotate and center". Now the size control is in the page set-up menu: You can try to set print size for example to 90%, then return to the print menu and check if margins are correct and nothing is cut-off. This works for many files of the orchestra-parts project. Some library scans are pdf's converted from 72dpi jpg's with hugely inflated page sizes - for these you have to try much smaller print sizes.
Required: an oversize printer with a double-sided printing option - many copy-shops will be able to offer booklet printing.
Things to watch to avoid booklet printing problems: Many files on imslp do not include covers - you will have to insert empty pages as space holders (using freeware like pdfSAM) for correct placement of left and right pages.
Sizing options are more limited for the booklet option in the Adobe Reader print menu ("fit to printable area" cannot be changed, always adding additional margins). An alternative that does not add additional margins is the free Foxit PDF Reader (for Windows, OS X, Linux and Citrix). See also Discussion
Alternative: Change margins in the pdf file: you have to use an image-processing software (Photoshop, Gimp or similar) to crop pages to the margins you like (this may require a lot of time).