Free for Windows
Reading through the literature leaves a dozen viewer windows open. Every title reads "Microsoft Edge" or the first twenty characters of a filename, and which window holds the paper you decided to cite — or the one that contradicts it — lives only in your head. Reopen them tomorrow and that structure is gone. WindowPin pins a label and a note onto each window, and puts them back when the window returns.
No installer · a single executable · touches nothing in the registry · v2.6.0
Put a note beside the PDF window with the sentence you want to quote, the page number and your own reading of it. Move the window to another monitor and the note travels with it, so you never have to work out which note belonged to which paper.
Label them by role — "key citation", "counter-evidence", "method reference". Twelve windows in a row still tell themselves apart by colour and name.
Close the window and the note survives. Open the same file again and the app recognises the window and re-attaches yesterday’s note, so you are not hunting for where you stopped.
Does it write annotations into the PDF?
No. The source file is never touched; notes are stored only in %APPDATA%\WindowPin.
Can it tell apart several windows of the same app?
It uses the process, window class and title, plus when the window was created. When the evidence is thin it asks you to pick rather than attaching to the wrong window.