If we have added a default custom field for an invoice, will that custom field be reflected when we convert that sales invoice to any other document on mobile?

No, the field will be visible only in selected document types. If we have set it as the default for invoices only, and if we convert it to any other document, the field will not be reflected.

If you want to reflect that custom field to other documents as well, please follow the below mentioned steps:

Step 1: Go to More >> Scroll down and choose custom columns

Step 2: Select the item for the custom field you want to reflect in other documents as well.

Step 3: Select All, It will be visible in all documents and click on Save

You can also select multiple document types, shown in the reference image below