I like your software and hope that you take this or something like this into consideration for future updates. I frequently work with 5 to 10 PDF's at a time and it is a pain to only have the option of single instance or all separate. I know that I can open 2 instances and open additional PDF's in these instances by using File/Open/ but it is clunky to do so and sometimes not possible with some of the programs I use. Basically the same as all major web browsers. Newly opened PDF's can be opened in either the original instance or the new instance and the user would have the ability to drag the PDF's tab over to the instance of their choice. What I would really like to see is the ability to open multiple PDF's in one instance, click on a single PDF's 'Tab' and have the option to open that single PDF in another instance. For 75% of the PDF's that I open I would like them to all be opened in the single instance and view one at a time, so the option to 'Allow Multiple Instances' doesn't really work for me. Foxit is great and in my opinion it is the best PDF viewer for general use, but not having the ability to open a file in a separate window is a little frustrating. I have been using Foxit Free for years and about a year ago I switched myself and my company to the paid Foxit PhantomPDF. I know that this is an old post, but I wanted to add my 2 cents.

Open two of the same pdf