Following on from communication in comments you seem to be asking about standalone executables. I.e. if a user has downloaded a zip file containing an application, extracted it directly to the Downloads folder (i.e. not in ~/Downloads/Program/).
Assuming a loaded dll is in the same directory then if it were to be replaced with a malicious copy odds are that the exe would load it without realizing. There are mechanisms - such as signing - to avoid this risk. Its also worth noting that to replace the file from most mainstream browsers without user consent should not be possible. Not to mention if you can replace the library why can't you just replace the exe?
When an executable attempts to load a library if an explicit path/manifest is not given the system is going to search a list of potential locations in a somewhat standardised order. This is documented for windows here - https://msdn.microsoft.com/en-us/library/windows/desktop/ms682586(v=vs.85).aspx
If for some reason an executable attempts to load a non-system shared DLL that it isn't shipped with then there may be a higher risk. The directory the executable is in would usually be the first search location. This means that should this DLL be injected into the downloads directory then it would be picked up. I suppose an example might be a program that only offers certain features if another software package is installed - for example offering Microsoft Work integration if Office is installed.