Maximum file size
In current version the maximum file size is limited to 2097152 bytes (2 Mb). In the next version this limitation will be lifted.
Working with versions
In case you turned on Version History in the Document Lbrary you will not be able to view previous file versions after you overwrite file with a different extension (i.e. "Document.docx" is overwritten with "Signed.pdf"). Since you have a new file extension SharePoint desides that all versons are also with new extensions. In this case if you try to download the previous version (before overwriting) you will get it with new extension. In fact, it is the previous version but with wrong extension.
If you want to open the previous version with different extension do the following steps:
1. Choose file and click at the Version History button on the Ribbon;
2. Right click on the version you need and choose "Save as...";
3. Specify the folder where you want to download previous version file, change "File type" to "All Files" and type the correct extension after the file name (i.e. "Document" to "Document.docx").