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Where to put the files, is customer-dipendent, feel free to put them based on your requirements, and change the code accordingly...In the article, we used app data folder just as an example.

To get the result doc, after saving it you can re-open it from its path, doc.createStream is used with a custom stream (PDFStream) that should be implemented by you, if you need to handle stream operations by yourself.

Thanks for the information, is there any limit on how many documents can be combined at once??

Not exactly, but you need to take into consideration documents size to avoid memory issues.

so you are saying that if i intend to combine 20 files each having 20 MB size then it shouldn't be a problem but if each file is 100 MB then it causes memory issues right ?

It depends also on the device and its available memory, also in case of 20 files you may face memory issues if the device's available memory is not enough.

So you need to give it a try, try different cenarios, and based on the nature of your app and your users .. you can put the limit.